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.
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:
da DUM da DUM da DUM da DUM
× / × / × / × / Come live with me and be my love
× / × / × / × / Dies Bildnis ist bezaubernd schön
× / × / × / × / Adon Olam Asher Malach
(the opening line of Adon Olam, a traditional hymn of anonymous authorship from the Jewish liturgy.)