Categories will refer to the type of post it is – an article, a link, a video, an exercise etc. It also refers to the specific skill (for example, grammar, writing, or listening), that is relevant to the post.
Tags refer to the topic it is about (for instance, Unlock 3 Unit 1: Animals).
Many posts are short videos that are meant to introduce, or incite further discussion into the topic. They are therefore not labelled under Listening or Speaking.
The best way to navigate the site would be to click on the appropriate tag, and see if there’s a link that you might be interested in. Alternatively, a search for keywords will do the trick.