Design patterns are general solutions to common problems that software developers encounter when building applications.
Think of it as just like an experienced carpenter has a set of tools and techniques that they use to solve common problems when building a house, software developers have a set of design patterns they can use to solve common problems when building software.
These patterns are reusable, meaning they can be applied to different parts of an application or even different applications entirely.
While building an application or fixing a buggy code/problem one of the main important things is to get it done fast, by using patterns, developers can save time and effort by not having to reinvent the wheel every time they encounter a problem.
So design Patterns provide easy-to-recognize and use OOP solutions to common problems. They're inherently easy to maintain because many people are familiar with them therefore it provides a common language for developers to communicate with each other, making it easier to collaborate and work on the same codebase.
This is very similar to how Google works. Everyone knows HOW to google, so when you get a query like "How to make pancakes", you can very quickly use this common interface to solve a problem.
