What does it mean in other words?
for universal background checks—and there’s more where that came from.

THE CONTEXT:

Billionaire Michael Bloomberg already had the gun lobby in his sights. Now Bill Gates is donating $1 million for universal background checks—and there’s more where that came from.

Question

What does it mean in other words?
for universal background checks—and there’s more where that came from.

THE CONTEXT:

Billionaire Michael Bloomberg already had the gun lobby in his sights. Now Bill Gates is donating $1 million for universal background checks—and there’s more where that came from.

Answer

Bill Gates has a lot of money, "so there is more where that came from" , (that is, Bill Gates' bank account)