Cheating means you take or do something outside of the accepted rules. Specifically...

transitive verb
1: to deprive of something valuable by the use of deceit or fraud
2: to influence or lead by deceit, trick, or artifice
3: to elude or thwart by or as if by outwitting <cheat death>

intransitive verb
1 a: to practice fraud or trickery b: to violate rules dishonestly <cheat at cards> <cheating on a test>
2: to be sexually unfaithful — usually used with on <was cheating on his wife>
3: to position oneself defensively near a particular area in anticipation of a play in that area <the shortstop was cheating toward second base>
Before getting married, my spouse and I agreed that neither of us cared to know of the others' possible dalliances... and also agreed that such extra-marital affairs were, by definition, permitted.

So other than it's use as the idiom for extra-marital activity... it's about the same as "cheating toward second base"

In other words... If it's in bold... it's bad. If it's in green it's keen.