Those actions can include stuff like sharing secrets with someone you're attracted to outside your relationship; keeping your relationship with a person you're attracted to secret from your partner; and taking steps to hide or mask the depth or breadth of that relationship from your partner.
If you really wanna dip your toe in the micro-cheating pool of destruction, you can check out this Thought Catalog piece, which details a delectable 33 different ways "Your Boyfriend Is Micro-Cheating (And Totally Getting Away With It)."
They include him doing cool stuff like "obsessively checking another girl's social media feeds" and "purposely neglecting to mention his significant other in conversation."
Of course, you could just IGNORE ALL OF THIS and build a relationship on mutual trust, respect, and admiration. But WHAT DO I KNOW.