This is the percentage of a server tick that the server spends processing running procs and the work of sending map information to players. A value of 0 would indicate very little cpu usage. A value of 100 would indicate full cpu usage, which could mean that the server cannot complete all the necessary computations during a tick to finish in time for the next tick. In this case, timed events (such as sleep) may take longer than requested.
When deciding on a value for tick_lag, one could use this value to determine if the CPU is fast enough to tick at a higher rate.
The map_cpu
var is a subset of this, measuring only time used for sending map information.