Preload
Preload is the initial tension on the suspension spring when the bike is unloaded. (air forks have an air valve where you can add or release air to achieve the desired preload effect. )
When to increase preload: heavy load,stiffness preference, need maintain bike stability in aggressive riding.
When to reduce preload: light load, feels too stiff or harsh over bumps, if bike have excessive sag
Compression
Compression damping controls how quickly the suspension compresses when hitting bumps.
When to increase: rough terrain, stability needs
When to reduce: smooth trail, improve comfort,need more travel
Always take a test ride after making changes to feel the difference and fine-tune as necessary.