"Шестерка" - is the object with value 6 in some characteristic.
For example, we can call "шестерка" - the veneer with thickness - 6 mm. Of course, listener should know what is the speech about.

Also, this word have two slang meaning is Russia.
The first - the cheap and simple car ВАЗ-2106.
The second - the lowest position in gangster hierarchy.