Title absolute is the best form of title that you can have. In layman's terms it means that there is confirmed "good" title to the land. Good title means that it is traceable back so that the person from whom you are buying has the right to sell it.
There is more to it than this, but in the simplest of terms that is it. You would still need to check what rights other people may have over the land, for example rights of way, covernments and easements.
To be sold, the land must be on the digital land registry, or if it is not the seller will have to register it. This makes tracing back the prior ownership much easier.