What is the problem with server space?

Question

linux

Colleagues, I never really worked with Linux, mostly Windows, but, apparently, this situation needs to be corrected.
The situation is this: a RAID of 4 disks is raised on the server /dev/md0, where, as the case is transferred, backups are written (in fact, they have not been written for a year :)), I noticed that it is /dev/sde3full. So far, that's all I've been able to dig up.
Backups do not go due to lack of space. Does anyone have any idea what is wrong with the server?
How to check what it is /dev/sde3, or see where backups are actually written?
Thanks for the help.
In general, I plan to redo everything in the future, nas4free and all backups there.

Файловая система   1K-блоков Использовано    Доступно Использовано% Cмонтировано в
udev                 1979180            0     1979180            0% /dev
tmpfs                 398152         5728      392424            2% /run
/dev/sde3          951150236    902764608          92          100% /
tmpfs                1990748            0     1990748            0% /dev/shm
tmpfs                   5120            0        5120            0% /run/lock
tmpfs                1990748            0     1990748            0% /sys/fs/cgroup
/dev/sde2             536536          132      536404            1% /boot/efi
/dev/md0         17510161708   2108754592 14522332252           13% /media/storage
tmpfs                 398148            0      398148            0% /run/user/1018
tmpfs                 398148            0      398148            0% /run/user/1000

Checked:
log 22 meters
tmp 28kb

Answer 1

/dev/sde3/ is the root filesystem.
check folder sizes /tmp, /var/log

Answer 2

/dev/sde3 is the root filesystem, where /bin, /var, /tmp, etc. are. Check /var/log - something must have messed up the logs.

Answer 3

I will join the recommendation Andrey , often the logs eat up the place catastrophically quickly