The bees will be next to each other with a bit of space between then and they will appear to be moving in a rhythmic movement back and forth.
They don't always move in sync but they can often appear to be.
What are they doing? They're using their tongues to clean the outside of the hive.
They'll mostly do this outside the entrances of the hive.
This is a task that I often see bees do once the population is built up so in a lot of ways it's an indicator of a strong hive.
See the video of the bees washboarding below.