Issue with disk space usage

Issue with disk space usage
I have the following line in my "df -h" output:

Filesystem              Size    Used   Avail Capacity  Mounted on
/dev/ad4s1a             496M    495M    -39M   109%    /

What is the issue with having 9% excess utilisation? How can I find out what this partition is used for and whether deleting some files is recommended?
Thanks in advance

What's the OS name and file system type? I guess fs type is UFS... check this: Minimum Free Space
Be very careful with deleting data from the root partition.
I'd run find to get all files greater than 5MB and move them to another partition or remove them.
Don't wait too long.

Thanks that worked