Some apps are refreshing and working in the background a lot of the time. This is if a notification comes in you will see it. Also to keep information updated so you won't have to wait for an app to load when you want to use it. Some apps will also run in the background for operational purposes. Things like Facebook tend to work in the background a lot and can use a fair bit of your battery life. Apps you don't use you can put to sleep and some can be adjusted.