I have a macbook pro. my wifi disconnects automatically when I stop using it. How can I get it to stay connected?

Try wireless diagnostics ...

If you're using OS X Mountain Lion or later, you can use Wireless Diagnostics to easily identify and resolve Wi-Fi issues.

WiFi is turned off and disconnects during Sleep mode. It turns back on and reconnects when you wake it up. You could configure it not to Sleep, but then your time on battery will be greatly reduced. The screen backlight, Keyboard lighting, WiFi radios, are turned off during Sleep mode by default to conserve battery power.

See: https://www.lifewire.com/what-happens-when-mac-is-put-to-sleep-2260787