question from a beginner

[Ankhcccc]

I'm a beginner, and I want to know how this software achieves the function of image blending.

[hi5]

It's a broad question, but you may have noticed the layer section, by default on the right, has various blend options:

When using a number of tools such as paint, stamp, brush etc you will another blend option in the TOP section of the window where you can change the options on how to apply the tool.

Does this help?