It's a harmless message, rather a good news for you!!
While booting, by default, the root filesystem (filesystem containing mount point
/), is gone through a filesystem check using
fsck, this is the result of that check. If there were any error the filesystem would be mounted read only, rather than read-write.
Although this an assumption from my side, to be precise this is the
fsck output for the filesystem created on partition
/dev/sda5, as you can do
fsck on any disk backed filesystem but by default
fsck is not done on filesystems other than the one having the root directory as mount point.
About the blocks, the filesystem divides the while partition into blocks, blocks are the minimal operational unit (so also the storage unit) to a filesystem. While writing, filesystem writes in full blocks i.e. if a file's actual size is 200 Bytes, it will take a block size space in the filesystem, hence on the underlying disk. For
ext4, the block size is 4 KB.
In the message:
956436/30277632 files, 37421826/123093120 blocks
956436 is the number of inodes used from a total of
30277632(fixed at the time of filesystem creation), same goes for the blocks usage numbers.
Here "clean" after the partition means the partition is allright, there is no issue with it.