Food does still digest when laying down but the food when lying down doesn't digest as good or fast as it does when sitting up or standing up.
Our bodies were made to digest the food we eat while we are in an upright position and laying down right after eating can lead to digestion issues and indigestion.
So it's best to wait 30 minutes to an hour after eating before laying down to allow your body to digest food properly.
If you lay down too soon after eating then you can have heartburn and indigestion issues.
But your body still does digest some food while you're laying down but your body should be in the upright position to get most of the food digested before laying down.