A monthly close is the process of finalizing a company's books for a given month: every transaction categorized, every account reconciled to actual bank and card statements, before moving on to the next month.
A real close means the books for that month are done, not still being cleaned up two months later. That includes reconciling every bank and credit card account to the actual statement, making sure revenue and expenses land in the right period, and reviewing the resulting financials before calling the month finished.
Skipping or rushing a close doesn't cause a problem immediately, which is exactly why it's easy to let slide. The cost shows up later, compounded, when six months of loosely-closed books have to be untangled all at once ahead of a raise or a tax deadline instead of reviewed one clean month at a time.
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