Justin,
Erlang is an admentional measurement. Is a relation between the time of ocupation and the time of observation. 1 Erlang mean, for example what anything was ocupated for all the time of observation. Ex. If a telephonic call to remain 1 hour, began 09:00 and was finished at 10:00 hs, and the time of observation this event was exactly in the same time we say what this call carried 1 Erlang.