If you know how many calories (or other nutrients) you ate, then you can use Quick Track to quickly add those calories to your daily log without searching for foods or by creating a new Custom Food. This is a helpful feature when eating at a restaurant.
To log a Quick Track entry:
On your daily log, tap the + button next to the meal you're adding to
On the food search page, tap the + button in the top right corner
Tap "Quick Track"
Enter your nutrients
Tap the checkmark in the top right corner when done
Just like when creating a custom food, if you leave the calories field blank then it will be auto-calculated according to the macros you set. You can also leave the Name field blank. When the name field is left blank the item will take the name "Quick Track @ 11:09 AM" on your log where the time is dynamically set to the time when you logged the item.