What does the IP2 mean for a PA or mixer?

second intercept point,it is more high,more better

IP2 measured with two tones, f1 and f2. IM2 frequency is f1+f2. IP2 is usually much higher than IP3. It is a very important issue in direct conversion receivers since second order products can also appear at baseband along with the desired signal.

Why second order products can easily into the baseband along with the desired signal ? Is it the componets with f1-f2?

IP2 means Second Order Intercept Point.
Usually measured into "linear" region, and used as "figure of merit".
Formally a 2nd harmonic may by generated as doubling of a fundamental or by difference between 2 freq. (F1-F2).
When specify IP2 , a better nomenclature should be used, i.e. 2TOIP2 or 1TOIP2 (stand for two tones, output, IP2; etc...).
A more general characterizzation is the harmonic table (often used for mixers), where you may find all nTOIPn.