Tell us what composers (and preferably also pieces) were played, and we can answer better.
As a general thing (since we don't know more yet), I think all we can say is that it refers to being "stylistically correct," and/or grossly disobeying important things in the score.
Like: Using too much pedal in Bach. (And of course some people think using any pedal is too much.)
Or, doing Wagnerian crescendos and phrasings in Mozart.
Or, ending a Mozart slow movement with a dramatic flourish, punctuated by jumping up from the bench and throwing the arms in the air in triumph. I think most people would say that goes against "performance conventions."
Or, using "terrace dynamics" in Chopin. (What's terrace dynamics?? Well, it's a "performance convention."
But for certain types of music more than others.)
Or, consistently using tempos and/or dynamics that are different from what's written and different from what almost anyone in their right mind ever does.
Tell us what the pieces were, and we can say better.
P.S. For examples of not following "performance conventions," check out Glenn Gould playing just about anything but Bach. Or, in the minds of some, even Bach, maybe especially Bach.