An Acrobat is a form of entertainer whose routine revolves around feats of great dexterity, such as climbing, swinging, tumbling, walking across ropes strung between high points, turning flips and so forth. It's a very physically demanding form of entertainment that requires tremendous coordination and grace.
Acrobats have appeared several times in the setting of Dungeons & Dragons; the first Acrobat was an experimental variant class for Advanced Dungeons & Dragons 1st edition, which debuted in the original Unearthed Arcana. Closely tied to the Rogue, it didn't catch on, but a second attempt was made through the Kits system in AD&D 2nd edition, where it was once again depicted as a thief specialized in climbing and tumbling. After this less than impressive outing, the acrobat disappeared as a class, although in 3rd edition, at least, a high-Dexterity Bard with proficiencies in the right skills could easily be played as an acrobat.
The Acrobat is mostly known to the D&D fandom for the fact it was used as one of the character classes to appear in the D&D Cartoon, alongside fellow Unearthed Arcana classes the Barbarian and the Cavalier.