I imagine that your own text or the movie you watched (or whatever your source of information is) tells you the answer you are supposed to come up with. I hope that the study that I have linked to below (just look at the summary that comes before the introduction) gives the same inforamtion as your text...
Basically, the strategy of a reindeer herd is to clump up. This is especially true of females with young and of reindeer herds in open country. When the reindeer clump up, the reindeer in the middle of the herd are much less likely to be bitten by the flies. This is actually quite important to their health.
Another strategy the reindeer use is to hide in the forest. This tends to be more of a strategy that is used by males since they are more likely to be on their own than females are.
Both of these strategies are believed to work to decrease the problem the reindeer have with the flies.