Optimization is the process of keeping a Mac hard drive in good condition by cleaning up files, defragmenting scattered data, and scanning for issues. Fragmentation occurs when files are divided into chunks scattered across the disk, slowing down the computer. Defragmentation reorganizes fragmented files into contiguous blocks to improve performance. The document recommends using the free Stellar Drive Defrag tool to optimize a Mac by defragmenting selected or all volumes, with features like temperature monitoring and email notifications.