The game saves your progress after you pay for a bed (the fee depends on how long you want to rent a bed).
Using a bed in a tavern - Taverns are located in most of the villages and towns.
Initially, you have only one "personal" bed but you can unlock more by progressing through the game and discovering more locations.
Using your own bed (the bed icon on the world's map) - The game saves your progress only after resting and sleeping for at least 1 hour.
Note - The game won't create a new automatic save if you didn't start another quest after completing a previous mission.
Starting a new quest or making progress in your current mission - The game will save your progress automatically.
This has a great impact on the gameplay and it forces you to play the game in a certain way. One of the biggest difficulties in Kingdom Come: Deliverance is that you can't save your game whenever you want.