A wildcard is a special character or symbol (* in the case of the NOP) that stands for one or more characters and is used in text string matching. This enables you to select multiple files with a single specification.

A few examples for ICAO codes:

EBB* returns EBBC, EBBE, EBBH, EBBL, EBBR, ...

*BX returns CYBX, EBBX, EDBX, LFBX, LTBX, ...

*BB* returns EBBC, EBBE, EBBH, EBBL, EBBR, ...

and also DABB, DBBB, ... and many more other combinations with a trailing or potentially starting (if such a combination would exist in the ICAO context) 'BB' letter group.