1. "Out and about" means exactly that: you are outside, from home, and travelling about, ie visiting or exploring.
2. s->th: this is a lisp. For some reason (the logic of which escapes me), when a woman speaks with a slight lisp, it sounds sexy. So some people decided to pronounce "sexy" with a lisp: "thexy".
3. "iseh" = a very sloppy way of saying "is it?" You've probably noticed some British dialects put the "t" at the end of a syllable at the back of the throat.
The phrase "is it because I'm black?" was made popular by Ali G, as a response to a rejection or refusal. It's probably quite complex to explain exactly how the comment works (also because it's on the touchy subject of racism), so whenever you see this glib quip, just assume the person is being a smart-alec.
The general rule with this quote is to say it in the poorest English possible.