To "tap into" something means to use it as a resource. Think of a faucet, which can also be called a tap. It is a way of getting a resource (eg water) on demand. We often refer to things being "on tap", or "tapping into" things in a figurative way.
It is difficult to get the full meaning of your quotation without the wider context, but it seems to be talking about someone with great influence over common people. "Tapping into their resentments" suggests that he knew the things that ordinary people were angry about, and used that to his advantage - perhaps agreeing with them in order to make allies, or making promises to fix things they were angry about. Politicians are often accused of doing this.