Constants are labels that have fixed values or macros that will not change (ever) during the course of your script. These labels are replaced at compile time by the Preprocessor. Constants are are not compiled into pcode, thus, files containing them should be recompiled with each execution.
Examples (quotes are mandatory):
Const "camera%" = "blitzscriptvarget(1)"
Const "PI%" = "3.14"
You can use Constants to tell the Preprocessor what to replace instead of modifying the Preprocessor's bscvmPreprocessor.ioi and bscvmPreprocessor.ioo files.