As you progress through Metal Gear Solid V: The Phantom Pain, you will continuously unlock new missions that you can embark on. Some of these missions are known as Side Ops.
Side Ops are smaller, quicker missions that are completely skippable if you so choose. They usually offer benefits to main missions by cutting off certain supplies, or granting you the opportunity to obtain valuable recruits. They also all come with a fairly decent GMP (money) reward. The fact that they’re usually short and sweet makes them perfect for players quickly trying to fatten their wallets.
Side Ops are fun, Side Ops are life, Side Ops get you paid.