It describes the fickle/unstable nature of things in this world.
The sutta compares:
– “rupa” to foam (bubbles that appear in a river and break quickly),
– feeling (vedana) also foam (bubbles that appear when it rains and break quickly),
– perception (sanna) to a mirage, and
– vinnana to the trunk of a banana tree (which has no “hardcore” or essence); one becomes tired at the end.
Feel free to ask questions if the translation in the link is not clear.