This read-only variable holds the unique identifying number for the instance. Every instance that you create is given a number that is used internally to identify this instance and this variable is what you can use to reference it. The id is also returned (and can be stored in a variable) when an instance is created using instance_create().
Note: The value of this variable is not the same as the identifier that the instance is given in the room editor and is also different to the instance_id array which contains all the ids of all the currently active instances.
for(var i = 0; i < instance_count; i++;)
if instance_id[i] != id
instance_id[i].scr += 5;
The above code adds 5 to the "scr" variable for every active instance in the room except the one running the code. It does this by looping through ALL the active instances (using the "instance_id" array to return each active instance's ID) and comparing them against the built in "id" variable, which is the ID of the original instance running the code.