In item 29 of Health Check what does # of holes mean? (4301802)

In item 29 of Health Check what does # of holes mean?

Here's the report produced:

Tablespace Fragmentation (1)
A common set of extent sizes is a good way to reduce tablespace fragmentation.
Sometimes fragmentation can be improved with 'Alter tablespace <tblspace> coalesce'.
Tablespace fragmentation should not be a problem if you use locally managed tablespaces.
======================================================================
== Tablespace Name % # of # of ==
== Fragmented Extents Holes ==
======================================================================
USERS 15 990 175