A dodecagon is polygon that has twelve sides, just like an octogon has eight sides.
The -gon part of dodecagon should clue you in that this is a geometric figure. That leaves the dodeca- part, which means 12. A dodecagon is a polygon, just like a pentagon (five sides) and a hectogon (100 sides). The Greek roots make 12 by combining do (2) and deca (10). This combination of roots can also be found in dodecaphonism (musical composition using the 12-tone technique) and dodecastyle (having 12 columns on a side, which describes certain antique architecture).