Net worth = assets − liabilities. Add up accounts, investments, pensions and the market value of your home; subtract mortgage, loans and card balances. The result can be negative, and for many young people with a recent mortgage it is.
It is the metric that corrects the illusion of salary. Two people on the same income can have opposite net worths, because salary measures the flow coming in and net worth measures what stayed.
The way to use it well is as a time series, not a snapshot: record it once a quarter, always on the same basis, and watch the slope. The absolute value depends on your age, your country and your luck. The slope depends on your decisions.