A "penis pump" is a device marketed as either a masturbating machine or a penis enlarger. It applies a partial vacuum to the penis. Beware – I have heard nothing but bad things about them from doctors, so JackinWorld definitely recommends that you stay away from them.
Think about it – your penis is very important to you, right? Why risk injuring it by subjecting it to an artificially created strong vacuum? I certainly would never put my own penis through these harmful conditions, and neither should you.
The same thing goes with using a vacuum cleaner as a masturbation aid. Never do this! You could hurt yourself! Be kind to your penis, and it will provide you the lifetime of pleasure you deserve as a human male.