what is extra offset?

Kinda.

The extrapolation offset is a value determined by network stability to smooth lossy connections.

So having high variance or jitter can cause this value to increase.

The factors to pay attention to are latency, latency variation, and packet loss.

Packet loss will ideally read as zero, latency variation will be less than 5 on a good stable connection and you want your latency as low as possible.