Broad match keywords show your ads for loosely related searches — including completely irrelevant ones. If you are selling office chairs and bid on "chair", your ad shows for "baby chair", "hair chair", "wheelchair" — wasting your budget on zero-intent clicks.
Fix: Use a mix of phrase match and exact match keywords. Start with exact match for your most important keywords, add phrase match for variations, and use broad match only with Smart Bidding once you have conversion data.