Trimeter is a metrical line made up of three feet. It can appear in different patterns, such as iambic or anapestic trimeter, depending on which kind of foot is repeated. Poets use it for a shorter, quicker rhythm than longer meter lines.
Example
The river moved
through reeds and stone;
The moon rose slow,
alone.
When here // the spring // we see,
Fresh green // upon // the tree.