The CHECKPOINT statement forces all the dirty pages for the current database in memory buffer to be written to disk.

Dirty pages are data or log pages (a page is 8K) modified after entered into the buffer cache, but the modifications have not yet been written to disk.

You would use CHECKPOINT after you shrank the database using DBCC. Without checkpoint you may not be able to see the results of the shrinking until the server does an automatic checkpoint.

Related article:

CHECKPOINT (Transact-SQL)