Trochaic tetrameter is a meter in poetry. It refers to a line of four trochaic feet. The word "tetrameter" simply means that the poem has four trochees. A trochee is a long syllable, or stressed syllable, followed by a short, or unstressed, one. Stresses on a syllable are detected by simply noting which syllable one puts stress on when saying the word. In many cases, this is the syllable which is pronounced loudest in the word, for example, the word 'purity' will take a stress on the first syllable and an unstress on the others.