download sources is to help you download the source code of the jar package that the project depends on. It is mainly used for debug testing and viewing the source code. If you do not download, it is all decompiled code, excluding comments, etc., which may cause more troublesome problems in debug.
reimport is the cache used to rescan items and update intellij, which is generally not required. But in some cases, such as someone else has added a module, you may need to manually click reimport to let intellij scan the new module when you pull the code. It can be understood as re-importing the project.
generate sources this is used to generate source code into the target directory, which should also include some templates under webapp or js, css, and so on.