Chemical reactions are when substances change into new substances. The starting materials are reactants. What you get are products. The arrow shows the direction: reactants → products.
Atoms aren't created or destroyed in reactions - they just rearrange. That's conservation of mass. The same atoms you start with end up somewhere else.
Types of Reactions
There are several main types: synthesis (A + B → AB), decomposition (AB → A + B), single replacement (A + BC → AC + B), and double replacement (AB + CD → AD + CB).