Ubuntu versions 15.04 and newer perform a quick file system check on the root disk and report the result on the screen.
The message
/dev/sda2: clean, 286631/6111232 files, 2586472/24413952 blocks
has the following meaning:
The partition that was checked is "/dev/sda"
The file system is "clean", i.e. there are no inconsistencies
"286631/6111232 files": The file system has been created to allow a maximum of 6,111,232 files, and currently there are 286,631 files,
"2586472/24413952 blocks": The storage capacity of the file system is 24,413,952 blocks (probably 4096 bytes each), and 2,586,472 blocks currently are in use.
This is an informational message confirming that the file system is ok. There is nothing that needs to be resolved or repaired.
See the link https://answers.launchpad.net/ubuntu/+question/293967. There you will find a more information.