The modLoaded Preprocessor only executes a script, if a certain mod is present.

You call the modLoaded Preprocessor by adding #modloaded modID to your script file, with modID being the modId you want to check for:
Example: #modloaded minecraft

You can also provide multiple modID's:
#modloaded minecraft tconstruct will only be executed if minecraft AND tconstruct are loaded.

You can also invert a mod condition so that the mod is only loaded if the mod is NOT loaded: #modloaded !tconstruct minecraft will only be executed if minecraft is present and tconstruct is NOT present

If you added this preprocessor to a script, it will only be executed if the provided modID's are present, in other words if the respecting mods are loaded.