The editor number is a value between 1 and 32767 that is used by level editors to identify things placed in a map. In the DECORATE format specification, this value is listed at the end of an actor's declaration, before the actor block begins. It is also possible to define them in MAPINFO, through an editor number definition block, which is the preferred method nowadays and the only one available for ZScript actors. Internally, a value of -1 for the editor number means that the actor cannot be placed in a map.
In DECORATE, the editor number is, along with the SpawnID, filtered through the Game property to avoid conflicts. There is a list of standard editor numbers for reference, giving the editor numbers of all standard actors.