Skill stacking means combining several different skills that you are reasonably good at, instead of trying to become a master at just one skill. The combination of your skills can be much more valuable than any single skill on its own.
For example, someone might be: good at drawing, interested in interior design, good at communicating with people, familiar with social media. Individually these skills are quite common. But when combined, they can create a unique skill set. This person could, for example, become an interior designer who is especially good at presenting their designs online and communicating ideas visually to clients.
Skill stacking can also be useful because skills from different areas can support each other. For example, learning photography can improve your understanding of composition and lighting, which can also help with interior design.
The main idea is that your combination of skills makes you different from other people. You don’t necessarily have to compete by being the absolute best at one thing.