This is a meta-template.
It helps other templates detect what type of page they are on.
It detects and groups all the different namespaces used on Wikipedia into several types:
main = Main/article space, as in normal Wikipedia articles. talk = Any talk space, such as page names that start with "Talk:", "User talk:", "File talk:" and so on. user, wikipedia, file, mediawiki, template, help, category, portal and book = The other namespaces except the talk pages. other = Any namespaces that were not specified as a parameter to the template. See explanation below.
For backwards compatibility this template also understands the old name image for file. But using image is now deprecated.
Note! For most usage cases it might be better to use the simpler namespace detection templates.