The lyrics to "Know Your Rights" are a very sarcastic look at oppression of the poor and working classes via limiting their human rights. Joe Strummer likely wrote them as a sarcastic response to a series of public service announcements in poor areas reminding the civilians of their basic human rights - this would certainly explain the "This is a public service announcement... WITH GUITARS!" introductory line.
According to the lyrics, people only have three human rights:
1) The right not to be killed, unless it is done by a policeman or an aristocrat (perhaps referencing two recent incidents, the deaths of Sunderland boxer Liddle Towers (see also The Angelic Upstarts' "The Murder Of Liddle Towers
") and New Zealand schoolteacher Blair Peach in incidents involving police brutality).
2) The right to food money - as long as you don't mind "a little investigation, humiliation, and if you cross your fingers, rehabilitation," a possible reference to the stricter tests and investigations one had to pass in order to receive welfare payments in the UK at the time.
3) The right to free speech - "as long as you're not actually dumb enough to try it!"