What kind of applications use internet on the background?
Every Android app (including system apps) is a *NIX user and is assigned a unique UID within range 10000-19999 at the time of installation or during first boot (for pre-installed apps). That's how Android enforces Discretionary Access Control on apps.

The number you showed looks like UID of some app which has been removed now. You can check UIDs of installed apps from file /data/system/packages.list. UIDs near to 10000 are usually system apps or builtin Google/OEM/vendor apps.

If you have root, you can track/control apps' internet access based on their UIDs using Linux kernel's built-in firewall netfilter, which is controlled by userspace tool iptables. AFWall+ makes use of the same.