An "artist slot" is defined by one or more  primary artists on your album or single. This can be either a single artist name (for example "Rob Styles"), or a collaboration of primary artists (for example "Rob Styles & Chris Willis").
If you are using PlayTreks to distribute your music, you will see that your pricing plan has a certain amount of "artist slots" included. Our free plan, for example, has a limit of 1 artist slot.
If you ever need more artist slots, you can always upgrade your account.