what Jaques was doing at the forest? what relation he had with duke senior?

Jaques is a lord attending on Duke Senior. He has gone to the forest to contemplate life. Jaques is perhaps the premier character for showing how Arden is a projection of the self. Jaques tells Duke Senior, "A fool, a fool, I met a fool i'th' forest, / A motley fool - a miserable world! - As I do live by food, I met a fool" (2.7.12-14). In reality he has met himself in the forest. Arden again is projecting his own attributes. This is further strengthened by the fact that Touchstone is a mirror for other people. Jaques "foolishly" then wishes to become a fool because licensed fools were allowed to say anything without fear of punishment.