What does the welsh word amdanat mean?

It is the form of the preposition "am" when referring to the second person (singular or familiar): amdanat ti.

E.g. Rhywbeth amdanat ti = Something about you

"Am" is a very flexible preposition, and can mean "about", "for", "at", "around", "on", "towards" or various other things, depending on the context.

Prepositions are one of the most difficult things to deal with in any language. "Up" in English is a notorious example: you can go up, put up, put up with, set up, be up by, get up, slip up, end up in, and so on!