Blur more than one shape enough that their colours blend into each other. Potentially play with blending modes to change the effect.
This is a popular trend in website design especially. So you can use them to be trendy. They help to add colour to a design but they’re not solid enough to attract too much attention.
I’ve named this pattern “blob gradient” because there are a few ways to approach it, but it’s generally the result of blobs of blurry colour interacting.
There’s a lot of tweaking you can do, and methods you can use to achieve this sort of effect. Some of them are not simple and will take lots of tweaking until you’re happy.