Gross pay is what you earn; net pay, or take-home pay, is what you keep. The gap between them is taxes and deductions: federal and state income tax withholding, Social Security and Medicare, health insurance premiums, 401(k) contributions, HSA or FSA deposits, and anything else taken out before the deposit hits.
The gap is bigger than most people expect. A $100,000 salary paid semimonthly is $4,167 gross per check. Subtract roughly $600 of federal withholding, $319 of Social Security and Medicare, $250 of 401(k) at 6%, $150 of health insurance, and $200 of state tax, and the deposit is about $2,650. Over a year that is $63,600 of take-home from $100,000 of salary.
Take-home is the number that matters for budgeting because it is what you can spend. Bills, goals, and safe-to-spend all have to fit inside it. Gross is the number that matters for comparing jobs and for tax planning.
The common mistake is building a budget on gross. The second is forgetting that not all deductions are losses: 401(k) and HSA money is still yours, just parked. A useful way to read a pay stub is in three parts: taxes (gone), benefits (spent on your behalf), and savings (moved, not spent). Only the third part counts toward net worth.