Abraham also complained to Abimelech
Possible meanings are 1) Abraham was complaining about what happened or 2) "Abraham also rebuked Abimelech"
concerning a well of water that Abimelech's servants had seized from him
"because Abimelech's servants had taken one of Abraham's wells"
seized from him
"taken from Abraham" or "had taken control of"
I have not heard of it until today
"This is the first time I have heard about it"
Abraham took sheep and oxen and gave them to Abimelech
This is a sign of friendship and that Abraham agrees to make covenant with Abimelech. (See: rc://en/ta/man/translate/translate-symaction)