Constants are variables whose values never change.
They are helpful in documenting what values mean in different cirumstances.
To define constants in GameMaker use #macro.
#macro GRID_SIZE 20
Constants are not variables and values and therefore you do not assign values to them with =. You can imagine GameMaker performing a find-and-replace of the constant name with the value specified, as this is akin to what the compiler actually does.
To identify constants in code and separate them from variables, we use ALL_CAPS.