It means the end. Example: It's curtains for you if your company has gone bankrupt.

If it 'is curtains for somebody', they are in a situation without hope or that you cannot escape from. (it is an idiom) (= to be the death, end, or ruin of someone or something.) Examples: - If they can’t get into the export market, it’s curtains for the whole company. - When I saw he had a gun, I thought it was curtains for me. (It is from the lowering or closing of the curtains at the end of a stage performance, as Zaldy says.)

In that sense curtains refer to curtains in a stage show. So when it's curtains or curtain time, it usually means the end of the show.

An idiom.It means that you've reached(experience) a lot of trouble and difficulties.