Sure. The idea here is simply that much of the benefit of using these clean technologies doesn't go to the person who invests in the technology. An externality is when one person's action affects somebody else. A smoker's second-hand smoke affects the person next to them. A firm's pollution affects the people next to them.
The idea of a carbon tax is to do what we call “internalizing the externality”. Market prices don't take into account the harm done by the pollution. If the tax is set a price that's comparable to the harm caused by the pollution, that now becomes part of the decision-making process of the firm. That cost is internalized, and it now becomes part of their decision-making process—the goal of the carbon tax.