A DNS lookup typically refers to the process of converting easy to remember names called domain names (like www.google.com) into numbers called IP addresses (like 192.168.2.1).
Computers use these numbers to communicate with each other on the Internet, but these numbers would be difficult for humans to remember and can change from time to time when network configuration changes are required.
A great way to think of a DNS lookup is similar to the contact list on your phone, but a special one where it has everybody’s name without them having told you their number, and if they get a new number, your phone automatically updates it. You don’t need to remember each of your contact’s numbers, but searching for their name is quick and easy. When you select their name to make a call, your phone will automatically use their current phone number.