In some countries/airports, some or all of the bags may go through scanners before being delivered on the belts.
If the scan reveals anything suspicious they may mark the bag for customs officers to verify the contents in your presence (scanners are not foolproof, so there may be false positives where they flag your bag for inspection even if there is nothing problematic).
Depending on the procedures (and what has been detected), this may involve marking the bag, or just advising customs officers if they want to be able to monitor you without raising your suspicion.