There's no RAW answer.
Freedom of Movement is an infamously vague spell:
This spell enables you or a creature you touch to move and attack normally for the duration of the spell, even under the influence of magic that usually impedes movement, such as paralysis, solid fog, slow, and web. The subject automatically succeeds on any grapple check made to resist a grapple attempt, as well as on grapple checks or Escape Artist checks made to escape a grapple or a pin.
The spell also allows the subject to move and attack normally while underwater, even with slashing weapons such as axes and swords or with bludgeoning weapons such as flails, hammers, and maces, provided that the weapon is wielded in the hand rather than hurled. The freedom of movement spell does not, however, allow water breathing.
There are a few things that FoM explicitly allows you to ignore:
- Magic that impedes movement
- Grapples and pins
- Impedance from water
There are a few things that people commonly disagree on as to whether FoM allows you to ignore them:
- Encumbrance
- Speed reduction from armor
- Difficult terrain
- Squeezing
- Moving through an enemy's space
Neither WotC nor Paizo (the Pathfinder version of FoM is almost identical) have ever clarified how the spell is intended to function. All we have to work with is what's stated in the spell, and it doesn't explicitly address the above. As such, you (or your DM, if you're a player) need to make a judgment call as to the limits of FoM. The limits of the spell have to be drawn somewhere — it probably won't let you just phase through a wall — but it's not clear where they should be drawn.
If you read "enables you to move normally" as overcoming any and all effects that would hinder your mobility, you can ignore the speed reduction from encumbrance, move at full speed and charge through difficult terrain, and squeeze at full speed. At that point you need to decide where the limits are: can you slide through the bars of a jail cell without difficulty? Can you slip through the gaps in a chain-link fence?
Personally, I don't rule that FoM affects the latter set of bullet points, but there's nothing explicit in the text or FAQs I can point to. There's a reason that this is such a hotly debated topic, unfortunately.