Put simply, usucaption is a method by which ownership of property can be gained by lapse of time (acquiescence). While usucaption has been compared with adverse possession (i.e. squatting), the true effect of usucaption is to remedy defects in title.
The necessity for usucaption arose in Roman law with the divide between res mancipi and res nec mancipi. Res mancipi required elaborate and inconvenient methods to transfer property title (a formal mancipatio ceremony, or in iure cessio). Res nec manicipi could be transferred by traditio (delivery) or in iure cessio.
If res mancipi were transferred by traditio, full ownership would not pass and the recipient would become a bonitary owner. However, if the bonitary owner kept the res in his possession for a certain amount of time (two years for land, one year for chattels) his title would become full title and he could assert himself as dominus.