A Tristich is a type of poem that contains three lines. It can be a stanza or strophe and of course, poem. A stanza is lines that have reoccurring lines with a set meter. A strophe is actually a form of music. It is the "Chorus ode" of Greek descent. And a poem naturally is a form of writing that has musical and speech qualities. A piece of writing with a lyrical flow. A Tristich, typically, is a parallel verse. This means that they may sound similar in a few ways. They may have a similar meter within the three lines or construction. The sound may be similar or even the meaning may be similar. Whichever way, a Tristich may most commonly consist of these parallel additions.
A tristich is simply a poem that consists of exactly three lines. Traditionally, it is an ancient three line parallel verse much used in Hebrew Biblical poetry style.