Sales commission is the variable part of a sales rep's pay: money earned on attainment, on top of the base salary. It is usually either a percentage of the closed revenue you bring in or a flat amount per unit — for an SDR it might be a fixed fee per qualified meeting; for an account executive it is a percentage of the deals they sign. Commission is the second half of OTE, and it is the half that is not guaranteed: you only earn it when you hit your quota.
In plain English
Commission is the "money for results" part of the paycheck. Base covers you; commission rewards you for producing. A rep who hits quota earns the full commission the plan promised, and a rep who misses it earns less or nothing. How much and how fast depends on the plan — the rate, the accelerators, and the draw that covers you while you ramp.
Why it matters to you
Commission is what makes a sales career compound. It is also the risk in the job: it is not a guarantee, and the ramp period and any draw exist precisely because your first commissions arrive late. Before you accept an offer, read the plan — the rate, what counts toward quota, and what attainment is realistic — because the gap between the OTE on the job post and the commission you actually collect is where most of the disappointment lives.
For making a closed deal count, see After You Close the Deal.