Many times, rhythm and rhyme are confused. Rhyme is where the words used sound alike or similar (for example, how and cow). Rhythm is very different. Rhythm is the "beat" of the poem.
To best describe rhythm, think about your favorite song. The rhythm is what you can clap to. For example, let us look at a children's rhyme.
Mary had a little lamb, a little lamb, a little lamb,
Mary had a little lamb, whose fleece was white as snow.
The bold parts of the line is where the words are stressed and a person would clap. This creates the rhythm of the song.
For poetry, this is the same. A reader needs to examine where the emphasis is in each line according to their reading. Sometimes, problems arise when determining the stressed and unstressed syllables given that some readers may naturally stress words very differently than others. Regardless of this difference, the rhythm still exists.
In the end, rhythm is simply the "beat" of the poem.