After looking up what contract this Bernard Atton is mentioned, it seems that he had made a contract of Fealty to Lord Leo in exchange for a fiefdom. In other words, Bernard pledged his and his sons' loyalty to Lord Leo and his successors. This was in exchange for being in charge of a chunk of land that included a castle, several villages, and a monastery.
This was a standard vow of loyalty, drawn up by a monk, to ensure that Atton and his sons would stay true to Leo (and everyone after him) while managing a section of Leo's landholding. This also linked Attons to support Leo in any conflict and made sure that the Attons would not rise up against him. The Attons gained land and authority, as well as the privilege of supporting the monastery, which was looked at as a holy benefit.