What topics should go in the thesis Appendix?

I am not sure about some side-topics related to my thesis. During the development of the results I have to rely on some other mehods which are not the main topic of the thesis but since I used them to derive some approximation I then implement in the analysis I think it is useful to explain how this methods works so any reader can understand what I did and also because I spent time learning them.

I was thinking to put these extra-info in an Appendix not to cause confusion in the main topic, but I am afraid they will be considered as off-topics.