In this context? Same as ever: decide whether to appropriate and give permission to spend. But the spending itself is an executive branch function.
The function of Congress it to authorize. The function of the President is to decide whether to act on the authorization.
This is critical to separation of powers in the design of the US government. It's important, as it avoids conflicts of interests should Congress be able to demand executive function.
And if funding runs out this week, that function of Congress will be front and center.