Extraneous means not necessary or not belonging to what you're focused on. When something is extraneous, it's extra stuff that doesn't help and often just gets in the way.

In math class, when you solve a word problem, extraneous details are facts the problem gives you that don't actually help you find the answer. If a problem says “Sarah has 12 red marbles and 8 blue marbles, and her birthday is in June,” the birthday information is extraneous: it doesn't matter for counting her marbles.

When you're writing an essay about your summer vacation, a long paragraph about your friend's dog would be extraneous if the dog has nothing to do with your trip. Good writers learn to cut extraneous sentences that distract from their main point.

Scientists designing experiments work hard to eliminate extraneous variables, which are factors that might confuse their results. If you're testing whether plants grow better with music, you'd want to make sure both plants get the same water and sunlight. Otherwise, those become extraneous factors that mess up your findings.

Think of extraneous information as outside the boundary of what matters for your current purpose. It might be interesting, but it doesn't belong.