People might already know but I’m going to explain it anyway.
Yum bonus is just bonus bars you get when you are eating something, it gives you more bars at your limited bars. This bonus bars didn’t add one square at your bar status, but giving you the sign +number at your bar status. That is yum bonus. When you do your activity, these yum bonus is used first instead of your bar status.
How to get? Simply, before you eat something, just makes sure that your emoji is at “yum” 
, not at “meh” 
. Every food you eat at “yum” will give you +1 bonus. However, this bonus might reset to 0 when you eat when your emoji is at “meh”. Means, you have to eat different kinds of food every time.
For example, you eating one berry, your yum bonus is +1. Then you eat one carrot, then your yum bonus now is +2, and so on. But if you eat berries again and your emoji is showing meh , your yum bonus will reset to zero. So makes sure you eat at Yum so you got plenty of bonuses.
Usually, you need to eat 4 different kinds of food before you can eat the same kind of food again. For example if you eat berries, and then carrot, and then pie, and then stew, you can eat berries again because the emoji will show you Yum.
I still do not know how many max yum bonus you can get. My record is +6 before i killed by a snake or killed by griever. Also, it might little hard to get higher bonus as an eve or early develop camp as not many kind of food around.
So, Don’t ever eat the same food twice in a row or you will not obtain this yum bonus. It’s quite handy as you can do more stuff than just desperately eating again and again.