The equivalent of barn raising, or bucket brigade. Widespread not only in Serbia, but in most of the ex-Yu countries, and probably Balkans in general.
Except it isn't limited to barn raising, or farming, or fire, or anything. It's for any occasion when you need a few able bodies to do some physical work for a day, and can't afford or wouldn't be bothered to find day labourers. Then you call your friends to help. Friends from work, your drinking buddies, anyone who isn't ill, or attending a wedding or a slava or a birthday or something that day (sometimes they'd appear at least for a few hours, despite other engagements they may have).
Most of the time, it's when you're building your home, or unloading materials for it. The trucker may not have workers brought along, and generally wouldn't be able to unload a few tons and still drive back. When you're building your house, there are usually a few days when you need to have a lot of concrete poured really fast, in one piece. That's called "livenje ploče" - pouring the board/slate/plate, which is the whole floorboard, or whole ceiling.
Before the moba arrives, everything should be more or less ready - sacks of cement not too far from the mixer, water and shovels available at the mixer, scaffolds or some kind of lift (usually with pulleys, but some rent a real elevator which lifts one wheelbarrow of ready concrete at a time), few crates of beer (mason beer accepted as fit for the occasion), and someone to bring lunch (usually beans with smoked ribs or knuckles, plus some salad on the side). And, not to forget, someone qualified to actually be at the front side of the concrete poured, to make sure it's even, flat, good quality etc.
How is this repaid? Generally, by not refusing an invitation to a moba when someone calls you. Most of the people who come to your moba may never call you, and those to whose moba you go may not come to yours, but it doesn't matter. Somewhere at the universe's scales it all evens out. Nobody's calling if they don't have to, so whoever calls surely has no other way, is a friend in need, so you just go, period.