Cracked Minecraft servers are all those servers that are running in offline mode, meaning Minecraft players can join using Launcher without authentication. If not handled by Cracked server itself, players can create unlimited number of accounts making Cracked Minecraft servers difficult to moderate.
What's the difference between cracked and premium servers?
Premium Minecraft servers usually uphold higher standards tho that's not always the case, you can find both good and bad premium and cracked Minecraft servers. You won't be able to join Premium servers without Minecraft account tho, but you will be able to join cracked Minecraft servers without Minecraft account. Some Minecraft servers even implemented support both Cracked and Premium players.