Template Comments is a debugging tool that wraps your Twig {% block %}s and templates that you {% include %} with HTML comments. By default, it does this only when devMode is on.

It also records performance data, so you know how much overhead each {% block %} or {% include %} is adding.

With more complicated "content builder" setups, this can help bring clarity to where the various HTML on your pages is coming from.

This can be especially handy when dealing with OPC (Other People's Code). It solves this problem.