What happens to lemon juice when boiled?

When lemon juice is boiled, it reduces, which means that water evaporates. This concentrates the flavor. This concentration and cooking also changes the flavor. The now cooked juice, while still clearly identifiable as lemon, will be much less bright. Keep going, and eventually you will get a syrup. Soon after that it will burn.

The main acid in lemon juice is citric acid. It’s a good inhibitor of oxidation, so lemon juice will slowly oxidate at room temperature or in the fridge. However, heat increases the rate of oxidation, the rationale here is as you heat the lemon juice the molecules will start to move faster thus increasing the rate of oxidation.

So in short: When you boil lemon juice, the molecules in the lemon juice will oxidize faster. It’s hard to tell how oxidation effects taste, but my experience as for the lemon is that, it will mainly impart a bitter taste.