A pip is how forex moves are counted. EUR/USD moving from 1.0850 to 1.0860 is a 10-pip move. USD/JPY moving from 150.20 to 150.30 is also 10 pips. Most brokers quote a fifth decimal, a pipette, which is a tenth of a pip.
Pip value depends on lot size and the quote-currency. It is the forex equivalent of tick-value.
Example: on a standard-lot of EUR/USD, one pip is $10. A 25-pip stop risks $250. On a micro-lot, one pip is $0.10 and the same stop risks $2.50.
Related: standard-lot, micro-lot, tick, quote-currency