Literally, a desk in a shop (both retail and workshop), a counter. In the most common usage, a gig on the side, most often off the books. In the days of SFRY, they took a lot of care to prevent anyone getting too rich by (ab)use of market circumstances or something the official economy failed to provide, so there were many roadblocks to freelancing, mostly in having very few ways to have the freelancers legally paid.
Ingenuity would win the day quite often, of course, and there were ways, intermediaries, loopholes, and there were tezgaroši, people who would jump from gig to gig.