WEP stands for War Emergency Power. It allows you to add more power to your plane's engine for a limited amount of time. IIRC, the P-47 used a water injection system to accomplish this. Other planes had other methods.
In AH you press the "P" key to enable WEP. It will cause your engine to heat up. When your engine reaches critical temperature, WEP shuts down automatically to avoid seizure. You can also shut off WEP manually by again pressing the "P" key.