It means any type of speech that is a victim of censorship opens a path to tyranny.
Look at Trump as an example. People are censoring him left and right out of fear and spite.
If you censor people you don't like just because of their political views or religious views or what-not it opens up a venue to censor others that follow. That mechanism is what dictatorships and authoritarians use.
It's interesting seeing these comments juxtaposed. Trump seeks a path to tyranny, and his unfettered speech was a threat to democracy. Free speech is an idea that arose in reaction to the British crown suppressing dissent among colonials who felt exploited and underrepresented. At that time, free speech was an asset to those advocating for democracy and a threat to tyrants. Today, it's the other way around.