You can disable the compiler in two ways
1) Go to System > Tools > Compilation page and click on Disable button
2) Change directory to the magento installation location .then type the below given commands
php -f shell/compiler.php — disable
php -f shell/compiler.php — clear
It allows us to compile all files of magento installation in order to create a single include path to improve the performance
Before compiling the code make sure that “includes” and the file “includes/config.php” must both be writeable.
You can compile the code from the admin panel System → Tools → Compilation
NoTE: Before you make any changes to your Magento installation you should always disable compilation. Once the changes are made, run the compilation process, and then enable it.