What does siente mean?

Without any context, "siento" can mean two things.

It is the first person singular present tense form of two different verbs, "sentir" and "sentar". So it can mean either "I sit" or "I feel". "Siento en la silla" is "I sit on the chair". "Lo siento por tu problema" is "I am sorry (I feel it) for your problem".