As Please says, it means "Thoughts are free," i.e., tyrants and fanatics can't prevent us from thinking what we will. It's an old German freedom song, that was produced in its current form in 1842. Older versions on that theme go back to about the 12th century or so.

Here is a link to subbed version of it, one of many traditional German songs put up on YouTube by Dr. Ludwig, who has been carrying on his own struggle against the censors:

https://www.youtube.com/watch?v=3h6ZZ28QtX4