In spite of its superficial appearance, this is not an ant. It is a two-banded antmimic spider (Castianeira cingulata).
It is a kind of jumping spider (family Salticidae). Most species of antmimics copy a specific kind of local ant. This one mimics the carpenter ant. It moves more like an ant than a spider. It will even raise its front two legs to mimic ant antennae.
It is thought that antmimics use protective mimicry to protect themselves from other spiders and from predators who eschew ants, such as birds. Some antmimics will also hunt ants themselves.
I found this one on my kitchen sink one afternoon last month. It spent several minutes exploring the adjacent counter, then went on its way.