Wikipedia defines freedom of speech as "a principle that supports the freedom of an individual or a community to articulate their opinions and ideas without fear of retaliation, censorship, or legal sanction."
But free speech has its limitations.
Legal Limitations of free speech
You can't scream "Fire!" in a crammed supermarket and cause a stampede without facing legal consequences.
There are laws against harassment and incitement.
In Austria there is a law called "Verbotsgesetz 1947" that e.g. makes it illegal to use the Nazi salute "Heil Hitler".
Free speech and the Fediverse
By some definition (almost) every instance of the Fediverse is a free speech instance, meaning there is no government censorship, but what is usually called a free speech instance is something else.
FS instances are infamous for moderating as little as possible. E.g. they don't block other instances, even if they are full of hate speech, but believe the users should block users and instances they feel threatened or harassed by themselves.
The thing is that by opening your instance to hate speech you indirectly are blocking reasonable people from speaking up on your instance, because they won't feel comfortable there.
That's why most FS instances are full of racists, misogynists, sexists and that sort.
To really allow the maximum of different opinions you have to block people who are oppressing and harassing other people, or who are spewing hate speech.
How to avoid free speech instances
There are instances that follow the, so called, Friendly Moderation Covenant of minimal moderation. Instances that follow that covenant federate with all kind of shady instances, so avoid those if you don't want to see Nazi posts or similar on your timelines.
Even worse are the instances on the FediBlock blocklist.
Before joining a Mastodon instance you can look at its /about/more page to see what instances they block - if the instance publishes that list. If that list is very short, there is probably no strict moderation on that instance.