For Want of a Nail (proverb)

The phrase "For want of a nail" is a proverb that means sometimes a little thing can cause big problems. It's like a chain. If you don't fix one small link in the chain, the whole thing can break and it can cause a really big problem. For example, let's say you have an important meeting to go to, but you forget to put on your shoe because the shoelace isn't tied. This one small thing (the nail in this proverb) can make things go badly for you and you could miss your meeting because you're walking to the meeting without a shoe. So "for want of a nail" means that a small thing can cause bigger problems.