To overlook something is to fail to see it. When planning a party, it's best to write out a to-do list so that you don't overlook any details.
The verb overlook literally means to look over the top of. The word has come to refer to the act of neglecting something, usually because your focus is aimed elsewhere. You might overlook the cost of car insurance if you’re busy calculating the cost of buying the car and paying for gas and parking. As a noun, an overlook is a space high above ground. You might overlook your curfew if you're busy enjoying an overlook with your date.