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 Log button in the bottom corner when done
Just like when creating a custom food, if you leave the calories field blank, it will be auto-calculated based on the macros you set. You can also leave the Name field blank. When the name field is left blank, the item will be named "Quick Track @ 11:09 AM" in your log, with the time set to the time you logged the item.