Combat is an integral part of Mount and Blade 2 Bannerlord. Hence, your character will often be injured. Health is not immediately restored after each battle - if you're injured and then join the next battle, your character will start fighting with a reduced level of Health. Here is a guide to how to heal yourself in Bannerlord.
How to restore health?
There are no magic, consumable health items in Mount and Blade 2 Bannerlord that would let you replenish health - an injured character needs time to fully recover. But you can speed the process up. In the bottom right corner of the screen there is a heart icon - hover over it to see the recovery rate. It's influenced by the Medicine skill level of (yours and your companions - you can hire a person with high Medicine to be your fletcher), but most of all, by your current location. The quickest way to recover is by staying in a city or a village for a while. Just enter a settlement and select Wait here for some time. Healing while traveling or camping in the woods is much slower.