I don't understand

Hello I am new member and noob about sysadmin I have one file server have HDD 2TB and install Directadmin When I try command "df -h" [root@server ~]# df -h Filesystem Size Used Avail Use% Mounted on /dev/sdb2 9.5G 9.0G 0 100% / /dev/sdb5 1.8T 196M 1.7T 1% /sdb4 /dev/sda1 452G 199M 428G 1% ...