The illustration on the left shows a document designed with bleed (finished size plus an extra .125" (3mm) on each side). Because minor variations can occur when cutting the paper to its finished size, to avoid being trimmed off, text and other important matter must stay .125" (3mm) away from the trim edge (the "safety zone").
The illustration on the right shows the printed piece after trimming. Note the last "e" in "Example" has been cut in half because it extended past the safety zone and into the trimming area. Some of the artwork has been cut off for the same reason. Any elements that extend past the safety zone risk being lost in the process of trimming the paper to its finished size.
You can read more about bleeds in our Learning Center by reading How to Add Bleeds to Your Print Job.