An employer match is a contribution made by your company to your workplace retirement plan such as your 401(k) plan or 403(b) plan. While there are a great number of companies not currently offering a match, any matching program is a great matching program and taking advantage should immediately become your most important financial move. Nowhere else can you receive a guaranteed rate of return on your money.
Say your employer matches 50% of the first 6% you contribute, a very common matching program. Let's further say your income for a pay period is $3,000. If you contribute 6% of your gross pay, you're 401(k) contribution will be $180 (calculated as 6% multiplied by $3,000). Since your employer matches half (50%) of your $180 contribution, it will add another $90 to your 401(k) plan. Between the two contributions - yours and your employers, your 401(k) plan just grew up $270 ($180 + $90). However, the cost to you was only $180. It's true cost was actually much less since you save taxes on the $180 contribution. No other investment opportunity exists with such an instantaneous and guaranteed rate of return as that 401(k) match.
Almost.
An even richer match is a "dollar for dollar match," or a 100% match. If you had a 100% match and otherwise faced the same circumstances as the example immediately above, your employer would contribute an additional $180 to your retirement plan - not $90. A dollar for dollar match is like winning at a casino every time you play. Why do anything else with your money? Take the free money only a 401(k) match provides.