You know the StackExchange takes the care of user-friendliness so-that user can get easily familiar with it! Just hover the mouseover the tab button and you'll see the tool-tip:
Frequent questions are those linked (i.e the url of the post is posted in question or in comment or th question is closed as duplicate) with other Q/A posts where as Active tab is just the real-time activity of posts.
For example the top most question of frequent tab is linked with 86 questions. You can investigate as follows: Visit What percentage of Hindus belong to a sect that subscribes to the Vedanta school? and you will find linked questions like:
Which shows the list of Q/A posts linked with the current question you're reading. You can see the full list of the posts linked with
https://hinduism.stackexchange.com/questions/linked/<post_id> e.g https://hinduism.stackexchange.com/questions/linked/3193 for the present question. You'll also find the number of question linked at the right panel.
Now, talking about the active tab, it is showing the latest activity of the post i.e recently asked, answered or edited. So, as soon as any new post is made or any existing post is edited it will be bumped to the active tab.
Following are some references would be useful: