In poetry, an iamb is part of a line that's made up of a short syllable followed by a long syllable. In an iamb, the stress is always on the second syllable.
Not all poetry uses iambs, but they lend a very specific, deliberate kind of rhythm and meter when they are used. Iambs show up in multiple forms, including "iambic pentameter," otherwise known as five iambs in a row, seen here in Tennyson's "Ulysses:" "To strive, to seek, to find, and not to yield." The stressed syllables in this line are "strive, seek, find, not," and "yield." One individual iamb is "to seek."