Net worth is the total value of everything you own minus the total of everything you owe. It is the only single number in personal finance that accounts for both sides of the balance sheet at once.
Assets
Anything you could convert to cash, valued at what you could realistically get for it — bank balances, investment and retirement accounts, property and vehicles at a defensible estimate, and any other holding large enough to move the total.
Liabilities
Every balance you currently owe, at its payoff amount rather than its original amount. A mortgage counts at remaining principal, a credit card at the current statement balance including spending you intend to pay off this month. Future obligations that are not yet debts, such as next year's rent, do not count.
Reading it
A single figure is close to meaningless in isolation. A negative net worth is the normal starting point after financing an education or a house. What carries information is the slope over a year, and which side of the sheet is producing it — rising net worth driven by falling debt is a different situation from rising net worth driven by an appreciating home you do not plan to sell.
Measuring it consistently
Update balances on a fixed cadence, monthly is plenty, so consecutive measurements are comparable. NetWorthy stores each balance as a dated snapshot, so a point in the history uses that date's balances rather than today's projected backward, and reports the asset and liability subtotals alongside the total.
See also How Net Worth Is Actually Calculated.