It is Harold Pinter's famous early play, written in the form of a tragi-comedy or a 'comedy of menace' as it is often called.
It was initially interpreted in terms of existentialism and the absurdist theatre of Beckett, whom Pinter has always been very fond of. But in recent times, the political sub-text of the paly seems to get more attention.
It is a play about menacing identity crisis e.g. Stanley. There is very little, almost no back-story at all.
It is a play about the social stereotype of madness and its ostracization in the asylum as that which happens to Stanley at the end when he is brainwashed and taken to a doctor Monty by the interrogators Goldberg and MaCann.
The play is about false communication, rupturous bonds and pointlessness.
It is about the irony of birthday and death-day. It is about the fluidity of the self and the conflict between the social forces of the big brothers and the individualist anarchist spirit of non-conformity on the other hand.
Considering Stanley as a pianist, the play may be seen to dwell on the great Modernist theme of the artist figure in his problematic relation to the society.
The play is also about the interior and the exterior and the arbitrary entrances with all their horrific aspects.