Bare spots happen on almost every lawn, sometimes from heavy traffic that causes the soil to become compacted. Whatever the reason, if the spot is larger than about a foot wide, it's important to patch it. Weeds will quickly fill the space or a deep rut can develop if the soil erodes without grass to hold it in place. Luckily, fixing bare spots is easy.
Try to use the same mix of seeds planted in the surrounding area so the new growth will match what's already there; however, there are other things to take into consideration. If your bare spot is from traffic, patch it with a seed blend meant for high traffic areas. If the bare spot is in a shady spot under a tree, use a shade mix.