Once you get the hang of it, it’s not too difficult to understand. I’ll try to make an example:
Not sure if you use Photoshop or any other 2D software but it’s actually pretty similar and it probably helps to understand the basic principles of the blending modes. Assuming you know a bit about layer blending in 2D software I’ll try to explain, but I’ll try my best to make it clear in case you don’t have prior experience with Photoshop
On a technical level you basically define how the content thats currently being rendered (your shader) into what has already been rendered on the screen (other objects inside your unity scene).
Alpha means: Blend the object based on a mask, in this case the alpha channel (so Alpha Channel will work like a mask in Photoshop)
Premultiply is a bit more complex: It multiplies the RGB channels with the alpha channel before applying standard Alpha blending. You might notice that the edges of your alpha being a bit smoother compared to regular alpha blending.
Additive means the pixels you render will be added to what’s already on the screen. Basically the brighter your RGB is the more visible it will be on the screen. This mode is often used to fake anything that glows.
Multiply in many cases does the exact opposite of additive Your pixels will get multiplied with whats already on the screen. And since all colors are normalized, you mostly use this mode to make things darker. Which means the darker your pixels are, the more visible they will get on the screen.
This is basically how it works.
Concerning blending modes in general I really like this article here:
It’s for AE, but the basics are the same. If there are any more questions, always happy to help out