Maypole dancing, like some other May Day festivities, seems to have started among the ancient Germanic peoples, though it became common in a lot of countries close to Germany and also spread to parts of America that have north European populations.
The maypole itself is probably a fertility symbol originally, and may also be part of the old Germanic tradition of sacred trees and nature worship in general. People carry out the maypole dance to celebrate the beginning of summer.
In Germany, especially parts of the south, the maypole dance is still a big festival. It can also be found in parts of Scandinavia. In the UK maypole dancing was banned by the church at different times in history, but it survived and has now become popular again. Each dancer holds a different coloured ribbon which is attached to a long pole, and the dancers make patterns by weaving around the maypole with their coloured ribbons.