From my limited knowledge in the area, I look for:
Reallocated Sectors Counts
Spin Retry Count
Read Error Rate
Uncorrectable Sector Count
These are some of the common S.M.A.R.T attributes that disk manufactures use to try and predict when a disk will fail. The 4 above are ones that are known to be critical to disk failure. Different disk manufactuers have different values for the attributes. CrystalDiskInfo reads the type of disk, then loads the manufactuers SMART attribute tables and compares that to the the values "stored" in the disk. When it finds issues, it flags warning in different colours (yellow as you have seen is a warning, red would be really bad).
You can read up more about SMART here:
http://en.wikipedia.org/wiki/S.M.A.R.T.
There are many other utilities that do a similar (possibly even better job). Perhaps some of the really knowledgable hardware guru's will also offer some comment, and correct any errors I have made.
Regards,
Golden