Iambic tetrameter is a meter in poetry. It refers to a line consisting of four iambic feet. The word "tetrameter" simply means that there are four feet in the line; iambic tetrameter is a line comprising four iambs.
Some poetic forms rely upon iambic tetrameter: triolet, Onegin stanza, In Memoriam stanza, long measure (or long meter) ballad stanza.
The term originally applied to the quantitative meter of Classical Greek poetry, in which an iamb consisted of a short syllable followed by a long syllable.
The term was adopted to describe the equivalent meter in accentual-syllabic verse, as composed in English, German, Russian, and other languages. Here, iamb refers to an unstressed syllable followed by a stressed syllable. A line of iambic tetrameter consists of four such feet in a row:
(Christopher Marlowe, "The Passionate Shepherd to His Love")
(Emanuel Schikaneder, libretto to The Magic Flute)
(the opening line of Adon Olam, a traditional hymn of anonymous authorship from the Jewish liturgy.)