IPTables can be tricky. The concept of chains pointing to chains pointing to chains can get complicated.
However, understanding the initial flow, the initial “which chain points where”, and the general concept which can allow, later, for easier NAT, or DNAT, or even knowing where to put a single rule is important. Especially if you are to utilise your Linux box as a router. Even if not, it better helps knowing how to define it.
So, here’s an image describing the common relationship between the predefined chains in Linux IPTables.