Comparative advantage is the idea that countries should do what they are relatively best at doing.
The idea here (my interpretation, anyway) is that even if you are better are doing everything, you're only (in the long run) going to end up better off in the free market doing what you're relatively best at doing.
How do you know what you are relatively best at? The book offers an explanation: if your productivity doing X is high relative to your productivity doing something else, then you have a comparative advantage in X. This is different from having an absolute advantage; if you are the only one capable of doing X, then you have an absolute advantage in X.
As a team lead, here's how I interpret this: a healthy team trades ideas, skills, and knowledge. In the short run, delegating tasks to team members according to their absolute advantage might pay off, but the problem is that nobody else learns from this experience. Say for instance, you're trying to introduce version control to your team. If you're the only one in the team that knows how to work with git, the easiest thing to do would be for you to set up the entire toolchain and processes up front, and train the rest of the team to use what you've set up. You'd get it done quickly and efficiently, but the team hasn't learned much from this exercise.
A better alternative might be to distribute tasks according to each team member's comparative advantage. Find the people that are more productive than you at teaching, and introduce them to the basics. Then ask them to onboard the rest of the team, and then you can get to work brainstorming the right branching strategy, choosing the most optimal CI toolchain, etc. This way is harder than the first and takes longer, but at the end of it, the team has grown both in skill and knowledge, and has learned to trust each other. And you've reduced the bus factor. The team is better off in the long run this way.