A quick google gives this; apparently from the creator.

"I[t] tells how fast your computer talks with lichess server. It measures the time a message needs to make the round trip from your browser to lichess, then from lichess to your browser.

I'm interrested in knowing what your ping is, depending on the country/city where you live.

I live in Le Mans, France, and my ping is around 30-40ms.

What about you?"

{source} http://nn.lichess.org/forum/lichess-feedback/whats-your-ping