A serial number is a unique code assigned to a software product, in this case, Photoshop CC 2018. It's used to activate the software and verify that it's a legitimate copy. Think of it as a digital fingerprint that identifies your software and allows you to access all its features.
In recent years, Adobe has shifted towards a subscription-based model for its Creative Cloud products, including Photoshop CC 2018. This means that instead of purchasing a perpetual license, you can sign up for a monthly or annual subscription that includes access to the latest version of Photoshop and other Creative Cloud apps. photoshop cc 2018 serial number