Optimum power calibration is like making sure you use the right amount of power when you are cutting something with a knife. If you use too much power, you might cut too hard and make a mess or hurt yourself. But if you don't use enough power, you might not be able to cut the thing at all.
People who make things like CDs and DVDs need to use optimum power calibration when they are writing (or burning) information onto them. This means they have to choose just the right amount of power to use. This can be hard because different CDs and DVDs are made from different materials and can react differently to the power used to write to them.
If they use too much power, the CD or DVD might not work well or might even get damaged. This is kind of like cutting too hard with a knife and making a mess. But if they don't use enough power, the CD or DVD might not be able to store all the information that needs to be written. This is kind of like not using enough power to cut something and not being able to finish the job.
So, just like using the right amount of power when cutting with a knife, using optimum power calibration when writing information onto CDs and DVDs is important to make sure they work well and store all the information they are supposed to.