For example, in this screenshot:
WSJT shows !Italy, !Serbia, !U.S.A. What does it mean?
That is something that is activated by "Show DXCC entity and worked before status" (selectable on the "Settings" > "General tab"). From the manual:
When this option is checked WSJT-X appends some additional information to all CQ messages displayed in the Band Activity window. The name of the DXCC entity is shown, abbreviated if necessary. Your “worked before” status for this callsign (according to log file wsjtx_log.adi) is flagged with a single character and a change of background color, as follows:
! Default color bright purple: New DXCC entity
~ Light pink: You have already worked this DXCC entity but not this station
Green: You have previously worked the calling station
It looks like the FT8 client you are using is parsing the callsign of people calling CQ, and displaying their DXCC entity prefixed by a !.