Hello, everyone, I want to use Sentinel-3 altimetric data to obtain the lake water level, but I am not clear about the meaning of each variable in the measurement. NC file after downloading, may I ask if there is any document or introduction explaining the contents of variables?

For information about the altimetry data you should better look at
Radar Altimetry Tutorial and Toolbox
There is a dedicated toolbox for it. Probably this serves your needs better.