Rhythm is the movement created by a repeated pattern of forms.
The best way to understand rhythm is to think of a song. Songs have rhythm when a piece of the song repeats. When listening to a song with good rhythm, we recognize the pattern and begin to expect beats.
An example of rhythm in product design is the Airbnb app. In this feed, each listing is displayed with a picture, price, location and owner avatar. When scanning the feed, you begin familiar with the rhythm and know exactly where to look for elements in the pattern, such as price.
A break in rhythm will appear more hierarchical. Think about a song. When a song has a repeated rhythm and the rhythm is broken, something quite special usually happens.
In the Twitter app, the profile feed has a rhythm and is broken by a section with suggestions of people to follow. This break appears more hierarchical and is a good way of grabbing your attention.