There may be 3 usual components to a software engineer's pay at a tech company:
Base salary
Cash bonus
Long term incentives
Base salary is the monthly payment for coming to work. It's usually fixed and is often quoted yearly. Cash bonuses may be paid quarterly or yearly and they are often a percentage of salary (5-30%) and usually depend on personal performance in the period. Long term incentives give something that may only be convertible into cash at a future date. These can take the form of restricted stock units, stock options or other similar payments. An amount will be allocated to you, usually once per year, and then sometime in the future (1-5 yrs) these units will be convertible into cash at the prevailing rate. This prevailing rate is determined by the performance of the company. In the case of the stock units, it's simply the stock price at the time they become yours (also known as vesting).