Check out the Player's Handbook, page 192:
Squeezing into a Smaller Space: A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it’s in the smaller space.
Creatures without Amorphous are usually governed by that rule.
As pointed out by @Adeptus, the rule is also available in the SRD (page 92) and the Players Basic Rules (page 71 in the print version, Combat > Movement and Position in the web version).