An anapaest is a metrical foot used in formal poetry. It consists of two short syllables followed by a long one. It may be seen as a reversed dactyl.

Here is an example from Cowper, a line with three anapaestic feet:

I am out of humanity's reach

Because of its length and the fact that it ends with a stressed syllable and so allows for strong rhymes, anapaest can produce a very rolling, galloping feeling verse, and allows for long lines with a great deal of internal complexity. The following is from Byron:

The Assyrian came down like a wolf on the fold
And his cohorts were gleaming in purple and gold
And the sheen of their spears was like stars on the sea
When the blue wave rolls nightly on deep Galilee.

An even more complex example comes from Yeats. He intersperses anapests and iambs, using six-foot lines (rather than four feet as above). Since the anapaest is already a long foot, this makes for very long lines.

Fled foam underneath us and 'round us, a wandering and milky smoke
As high as the saddle-girth, covering away from our glances the tide
And those that fled and that followed from the foam-pale distance broke.
The immortal desire of immortals we saw in their faces and sighed.