Net worth is the value of everything you own minus everything you owe. It is the single clearest number for whether your financial position is improving, because it captures both sides of the ledger at once.
Income does not appear in it directly, which is the point: a high earner who spends everything can have a lower net worth than a modest earner who does not. Tracking it a few times a year tends to be more useful than tracking it monthly, since short-term market moves add noise without changing the trend.