Sheila has $100 to spend. Sheila must choose if she should spend her money locally or not.
If Sheila buys from a locally owned business then $68 of her $100 will stay in the local economy, providing additional growth and benefit to her local community. If Sheila buys from a non-locally owned business within her same community, $43 of her $100 will stay in the local economy, providing additional growth and benefit to her community. If Sheila buys online or outside of her region, zero of her $100 will stay in the local economy, providing no benefit to her community.
You're spending your money on more than just the item you're buying... you're adding a building block to your neighborhood.
Your Money doesn't stay in the till for long though. Local business owners use the money to create a lot of economic activity in your neighborhood. They purchase from suppliers, many of whom are also local businesses. Local businesses pay taxes to the city, which builds and maintains the infrastructure we all rely on. Next time you see a fire truck drive by, or a police cruiser stop to help a motorist, thank the person who bought local so there are funds to support your city's infrastructure.