When becoming a Reseller you have the option to pick a bundle of 3 or 30 apps. The 3 app bundle includes 6 seats while the 30 app bundle has an unlimited amount of seats.
If you've chosen the 3 app bundle you'll be able to have 3 published apps at the same time. The amount of apps you can still publish before reaching this 3 app limit is referred to as "Apps left in bundle".
However, you might want to remove one of those apps from your bundle and replace it with another app. You can use your remaining seats for this. Each app you publish will use up one of your seats. The "Seats left in bundle" shows you how many times you can publish and/or replace an app.
The seats are reset each year.