If you are an Amateur Radio operator using WSJT-X software and come upon an unusual pattern of FT8 transmissions, including several other operators calling another station that often has an unusual call sign and often without any "CQ" call from the unusual station that you can see, you may be seeing a DXpedition using WSJT-X's "DXpedition mode".
Your own WSJT-X software must be switched into DXpedition mode and you must follow certain procedures in order to contact this DXpedition station.
Do not call the DXpedition station until you have done these things. And, most important, do not call the DXpedition station anywhere on the waterfall near where that station is transmitting. (Calls near the DXpedition station's audio frequency will not be heard by the DXpedition station and will cause interference for everyone else. Don't be "that OM" or "that YL". Learning about this special mode includes learning where on the waterfall to make your calls.)
The most obvious indicator that you're seeing a DXpedition station are sets of transmissions from the station that look something like this:
310 K1ABC RR73; W9XYZ <XY0Z> -08 370 N2QRS RR73; A3DEF <XY0Z> -02 430 K4TUV XY0Z RR73
Note that the station is transmitting on several closely-spaced audio frequencies (in this case 310, 370, and 430 Hz, though there may be fewer or more, and their location on your waterfall display will depend on where your rig's tuning dial is set), and that in two of the transmissions it is addressing two different stations each and that in those transmissions there is a semicolon (";") and the station's own call sign is in angle brackets ("<>").
On the other hand, you may have read somewhere about the DXpedition and decided to check it out, though you have no experience working DXpeditions with FT8.
In this situation, you will need to do two things before you attempt to contact the DXpedition using FT8:
Every DXpedition is different, but in most cases the operators have gone to extreme lengths to travel to remote and often unhospitable locations, to set up, to operate, and to try to remain sane and healthy for the duration of the project. We hams owe it to them to learn about their project and to follow their requests and expectations. Find the magazine article, the Facebook or Twitter announcement, or the Web site and read.
When you set WSJT-X to the frequency the DXpedition is using on the band you are on (which will not be a conventional FT8 frequency), you should see that the DXpedition station is transmitting on one or more frequencies between 300 and 900Hz. You will be instructed to make your replies to the station's CQs some distance above that range, probably above 1000Hz.
Good luck making contact! 73!