The decode's frequency in hertz above the dial frequency
The perceived clock differential between the local station and remote station, in seconds.
WSJT-X client name
The message payload contained in the decode
The protocol of the decoded message
The Modifiers field allows the equivalent of keyboard modifiers to be sent "as if" those modifier keys where pressed while double-clicking the specified decoded message. See the WSJT-X source link above.
Local station's perceived signal to noise ratio
Clock time in milliseconds since midnight
In order for a server to provide a useful cooperative service to WSJT-X it is possible for it to initiate a QSO by sending this message to a client. WSJT-X filters this message and only acts upon it if the message exactly describes a prior decode and that decode is a CQ or QRZ message. The action taken is exactly equivalent to the user double clicking the message in the "Band activity" window.
See WSJT-X source.