The title refers to the characters' attitudes towards their lives in general, but Jimmy's in particular. There is a sense that life is passing them by, that they are growing older without things getting better, that their relationships had possibilities at the beginning that they don't have now, and so on. All of these generate a generalize resentment/anger in the play.
There are also a number of socially oriented comments that indicate a kind of generalized historical anger, often about change. For example, early in the play Jimmy mentions reviews that are half written in French (in an English paper). This change makes him feel stupid. Alison's parents were very angry at her marriage, and the couple often took pleasure in crossing social lines.
But again, it is Jimmy specifically whose past is most marked by anger. In Act II he talks about watching his father die, and he says, "You see, I learnt at an early age what it was to be angry—angry and helpless. And I can never forget it."
This past anger is shaping his life, and their lives.