Base salary is the part of your pay you can count on. It is stated as a yearly figure, divided evenly across pay periods, and paid whether the company had a good quarter or not. Everything else, including bonus, commission, and stock, sits on top of it and moves around.
For planning, base salary is the foundation because it is the only piece that arrives on a predictable schedule in a predictable amount. A $120,000 salary paid biweekly is $4,615 gross per check, 26 times a year. After taxes and deductions, that might be $3,200 per check, and that is the number your monthly bills have to fit inside if you want the variable pieces to stay free for goals.
Base salary also anchors other numbers. Bonus targets are a percentage of it, raises are a percentage of it, and 401(k) contributions and employer match are calculated from it. A higher base with a lower bonus target is often worth more over time than the reverse, because every future raise compounds on the base.
The common mistake in negotiations is accepting a lower base in exchange for a bigger sign-on or bonus. The sign-on is one-time and the bonus is uncertain, while the base is forever and grows. When two offers are close, the one with the higher base usually wins on a five-year view.