What is loop0 and how do I e2fsck it?

What is loop0 and how do I e2fsck it?

EXT3-fs (sdd1): using internal journal
EXT3-fs (sdd1): mounted filesystem with ordered data mode
EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts:
EXT4-fs (sdb1): mounted filesystem with ordered data mode. Opts:
EXT4-fs (sdc1): mounted filesystem with ordered data mode. Opts:
Adding 4194296k swap on /dev/sdd2. Priority:-1 extents:1 across:4194296k SSD
kjournald starting. Commit interval 5 seconds
EXT3-fs (loop0): warning: maximal mount count reached, running e2fsck is
recommended
EXT3-fs (loop0): using internal journal
EXT3-fs (loop0): mounted filesystem with ordered data mode
(loop0) is mounted too many times. What is it? Is it important?
This is my fstab
#
# /etc/fstab
# Created by anaconda on Wed Nov 1 00:29:46 2000
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
UUID=9fac7ae7-9948-4612-88dc-e652fc4ceb73 / ext4
defaults 1 1
UUID=1daa52c7-a5da-464d-a4c7-2ee19ef017af /boot ext3
defaults 1 2
UUID=12649fb1-fd53-4558-8a2a-79692ada8b19 swap swap
defaults 0 0
tmpfs /dev/shm tmpfs defaults 0 0
devpts /dev/pts devpts gid=5,mode=620 0 0
sysfs /sys sysfs defaults 0 0
proc /proc proc defaults 0 0
/usr/tmpDSK /tmp ext3 defaults,noauto
0 0
/dev/sda1 /home1 auto
auto,noatime,defaults 0 2
/dev/sdb1 /home2 auto
auto,noatime,defaults 0 2
/dev/sdc1 /home3 auto
auto,noatime,defaults 0 2