Two-sided means having two sides. That sounds simple, but 'side' pulls in two directions and the word follows both. First, physically: paper printed on both faces is two-sided printing; a two-sided coin, a two-sided mirror. Second, and more usefully for you, it means an argument or a situation has two parts that pull against each other — 'The story is two-sided' means there is more to it than the one version you heard. Teachers say 'Every argument is two-sided' to mean both people have some reason on their side. This second use is where the word earns its keep, and it is very often used to gently disagree with someone who is too sure. A related use is about relationships: something two-sided goes both ways, both people give something — though for that meaning 'mutual' or 'both ways' is what people say far more often. Be careful with one thing: 'two-sided' is mostly neutral or fair-sounding. If you want to say a person is dishonest, showing one face to one group and another face to another group, English does NOT use 'two-sided' for that — it uses 'two-faced', which is a real insult. The two words look like twins and are not.