To all those here who talk about 'cheating'

I'm a noob and Ive started to notice people here keep mentioning people cheating on reps or sets.
What are you talking about exactly? Not finishing the reps/set or counting wrong? I'm 2 weeks into the Fundamentals, and in a week our trainer is away so he said hes going to put us in the regular WOD class.. Im concerned I wont be able to finish the reps, and I dont want to be considered a 'cheater'