Radical Title can simply be explained as the title to a country by, for example a sovereign Crown.
It is different from the ownership of land in a country by a person.
Radical title exists over, or 'on top of' other titles such as fee-simple (private property) title for example.
Radical title does not of itself extinguish underlying titles such as fee simple title.

A good description of radical title can be found in the purported High Cout case of MABO AND OTHERS v. QUEENSLAND (No. 2) (1992) 175 CLR 1 F.C. 92/014 - Aborigines - Constitutional Law - Real Property.

COURT High Court of Australia
Mason C.J.(1), Brennan(2), Deane(3), Dawson(4), Toohey(5), Gaudron(3) and
McHugh(1) JJ.
HRNG - Canberra, 1991, May 28-31; 1992, June 3. #DATE 3:6:1992

JUDGE2 - BRENNAN J.
45. There is a distinction between the Crown's title to a colony and the Crown's ownership of land in the colony, as Roberts-Wray points out (74) ibid., p 625: "If a country is part of Her Majesty's dominions, the sovereignty vested in her is of two kinds. The first is the power of government. The second is title to the country ...
This ownership of the country is radically different from ownership of the land: the former can belong only to a sovereign, the latter to anyone. Title to land is not, per se, relevant to the constitutional status of a country; land may have become vested in the Queen, equally in a Protectorate or in a Colony, by conveyance or under statute ...
The distinction between these two conceptions has, however, become blurred by the doctrine that the
acquisition of sovereignty over a Colony, whether by settlement, cession or conquest, or even of jurisdiction in territory which remains outside the British dominions, imports Crown rights in, or in relation to, the land itself."
Similarly, Sir John Salmond distinguished the acquisition of territory from the Crown's acquisition of property (75) Jurisprudence, 7th ed. (1924), appendix "The Territory of the State", p 554:
"The first conception pertains to the domain of public law, the second to that of private law.
Territory is the subject-matter of the right of sovereignty or imperium while property is the subject-matter of the right of ownership or dominium. These two rights may or may not co-exist in the Crown in respect of the same area. Land may be held by the Crown as territory but not as property, or as property but not as territory, or in both rights at the same time.
As property, though not as territory, land may be held by one state within the dominions of another."
Professor O'Connell in his work International Law (76) 2nd ed. (1970), at p 378, cited by Hall J. in Calder v. Attorney-General of British Columbia (1973) SCR.313, at pp 404-405; (1973) 34 DLR (3d) 145, at p 210 points to the distinction between acquisition of territory by act of State and the abolition
of acquired rights:
"This doctrine (of act of State), which was affirmed in several cases arising out of the acquisition of territory in Africa and India, has been misinterpreted to the effect that the substantive rights themselves have not survived the change."
The acquisition of territory is chiefly the province of international law; the acquisition of property is chiefly the province of the common law. The distinction between the Crown's title to territory and the Crown's ownership of land within a territory is made as well by the common law as by
international law. A.W.B. Simpson (77) A History of the Land Law, 2nd ed. (1986) distinguishes the land law rule in England that all land is held of the Crown from the notion that all land is owned by the Crown. Speaking of the mediaeval conception of materialism, he comments (78) ibid., p 47:
"This attitude of mind also encouraged the rejection of any theory which would say that the lord 'owned' the land, and that the rights of tenants in the land were iura in re aliena. Such a theory would have led inevitably to saying that the King, who was ultimately lord of all land, was the 'owner' of all land.
The lawyers never adopted the premise that the King owned all the land; such a dogma is of very modern appearance. It was sufficient for them to note that the King was lord, ultimately, of all the tenants in the realm, and that as lord he had many rights common to other lords (e.g. rights to escheats) and some peculiar to his position as supreme lord (e.g. rights to forfeitures)."
The general rule of the common law was that ownership could not be acquired by occupying land that was already occupied by another. As Blackstone pointed out (79) Commentaries, Bk.II, ch.1, p 8:
"Occupancy is the thing by which the title was in fact originally gained; every man seizing such spots of ground as he found most agreeable to his own convenience, provided he found them unoccupied by any one else." (Emphasis added.)