In basic terms, the more stereo crosstalk, the narrower the soundstage and the less of a stereo effect.
Stereo crosstalk is the L channel bleeding into R one and vice versa.
But it has to be pretty bad to be noticeable.
EDIT: okay, I came back after some reading..
MX4 Pro has -47.2dB stereo crosstalk. Not at all representative value, but by itself, apparently it just about doesn't degrade the quality of output at all. It has to be below -40dB or so to get into perceivable by ears territory.
Stereo crosstalk value often serves as a representative value for the quality of the DAC, but again, by itself it doesn't affect the SQ unless it's really really bad (Oppo Find 7a bad).