When the bees seem to form a fuzzy beard on the hive and hang out in a cluster it’s called bearding. This happens in strong colonies when the population is at its height and as the bees are storing and ripening honey.
To keep the hive at the correct temperature and allow for airflow in the hive, a small to large number of adult bees will hang out in the front, helping the internal temperature to stay cool. You might even see some fanning of their wings, pushing air into the hive on the hottest days.
Check out this website on bees to see a full article on bearding.
The temperature cannot get too hot in the hive for the brood (eggs and larvae) to develop into bees. I raised the lid a bit off the top and enlarge the entrance space near the bottom to encourage air flow.
This can also be a sign the bees don't have enough room and can also be a sign the hive is about to swarm. But the bearding in my hive isn't during swarm season (the spring) and a bit of bearding is totally natural for a colony during the summer. I keep trying to convince my wife my beard is totally natural in the summer as well so that she lets me keep it.