Delta compression is used to send only the difference between the last sent state and the current state.
For example if you synchronize a Transform. Scale x, y, z is most likely going to keep the same value.
Unity compares the state received by the client to the new state, if it is changed 1 bit and the data will be sent.
If it didn't change only a single bit will be sent.
When Unity detects that nothing has changed since the last state, no data will be sent at all.