Gross pay is what an employee earns for a period, before anything is taken out.
Net pay is what actually reaches them.
What sits between the two
Federal income tax, state income tax where it applies, the employee's share of FICA, and any voluntary deductions such as insurance or retirement contributions.
The business withholds these and forwards them. It is holding other people's money in the meantime, which is why payroll taxes are treated more seriously than an ordinary bill.
The business pays more than gross
Gross pay is not the cost of employing someone. On top of it the business pays its own share of FICA, unemployment tax, and usually workers compensation premium.
Budgeting from the gross figure understates the real cost of a hire, often by a fifth or more. See labour cost.