Geldum, translated as tax in the Phillimore edition, is more commonly (if inaccurately) known as the Danegeld. References to tax occur in the great majority of Domesday entries; the word Danegeld occurs only once (LIN S1).
The geld originated as an 'army-tax', instituted by Aethelred the Unready to pay Scandinavian mercenaries employed against the Vikings. Later it became an annual tax to finance the army and navy of the Anglo-Danish kings, levied on the hides (or carucates) at which estates were assessed for military and other services. It is often confused with the tributes levied to buy off Viking invaders, payments which, if the chroniclers were to be believed, were far more onerous than the geld and on one occasion exceeded the total value assigned by Domesday Book to the whole of England. As Maitland aptly observed in Domesday Book and beyond (1897):
Unless we are prepared to bring against the fathers of English history a charge of repeated, wanton, and circumstantial lying, we shall think of the danegeld of Aethelred [the Unready]'s reign and of Cnut's as of an impost so heavy that it was fully capable of transmuting a whole nation (Domesday Book and beyond, pages 7-8).
He then added:
William [the Conqueror] might well regard the right to levy a geld as the most precious jewel in his English crown. To secure a due and punctual payment of it was worth a gigantic effort, a survey such as had never been made and a record such as had never been penned since the grandest days of the old Roman Empire. For ... the assessment of the geld sadly needed reform (Domesday Book and beyond, page 4).
The relationship of the geld to the Domesday survey has since been central to most of the controversies surrounding the 'making of Domesday Book'. Whether or not contemporary chroniclers were guilty 'of repeated, wanton, and circumstantial lying' remains a controversial issue, recently debated at length by M.K. Lawson and John Gillingham. Lawson's contributions can be found in: 'The collection of danegeld and heregeld in the reigns of Aethelred II and Cnut', English Historical Review, vol. 99 (1984), pages 721-38; '"Those stories look true": levels of taxation in the reigns of Aethelred II and Cnut', English Historical Review, vol. 104 (1989), pages 385-406; 'Danegeld and heregeld once more', English Historical Review, vol. 105 (1990), pages 951-61; and Cnut: the Danes in England in the early eleventh century (1993); while those of John Gillingham are in: '"The most precious jewel in the English Crown": levels of danegeld and heregeld in the early eleventh century', English Historical Review vol. 104 (1989), pages 373-84, and 'Chronicles and coins as evidence for levels of tribute and taxation in late tenth- and early eleventh-century England', English Historical Review, vol. 105 (1990), pages 939-50.
Although the evidence does not permit a definitive conclusion, Lawson's view that the levels of taxation were indeed extremely high by later standards appears the more persuasive. It is, however, clear from the subsequent history of the geld, that such levels were not maintained. For this, see Judith A. Green, 'The last century of Danegeld', English Historical Review, vol. 96 (1981), pages 241-58.