A penis pump is a vacuum device that can help your penis get hard and stay hard for penetrative sex. The effects of using a pump will last about 10 - 15 minutes. This will vary with each individual. The results can last longer if you use a C-ring which helps maintain the erection by limiting the flow of blood from your penis.
A penis pump will not make your penis "larger" in the long term. The exception to this may be Bathmate type products which require a program of consistent and frequent application.