why if I open an app and open another app the first disappear

If I understand correctly, you can work on one app at a time; the other would be "standing by" in the background.

This occurrence may be from Apple's automatic resource handling routines in OS X, where if your system is low on RAM, then the computer will automatically suspend inactive programs. They will not technically be quit, and instead will be simply removed from memory to allow other programs the resources they need to run. This usually happens on systems with 4GB RAM or less, and will happen if you see the Memory Pressure graph in the Activity Monitor utility show high memory usage. The fix for this is to either close documents and programs you are not using, or to get more RAM to accommodate your needs. These days, 8 GB is a good amount to have, but 16GB will work better. Beyond this, you may not see much benefit, unless you use graphics and large data manipulation programs that gobble RAM.