what do I need to connect my macbook to my iMac so I can use the bigger screen

Which MacBook and which iMac do you have?

Use your iMac as a display with Target Display Mode - Apple Support

from Duane's link…

"Note: iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays."

You will not be able to just connect them with a cable.

You could try something like ScreenRecycler and see if it fits your needs.

There's also something called ScreenResX that is suppose to do what you want. I have no idea if it works myself.

Do you mean SwitchResX? That allows you to control resolution settings in great detail but doesn't allow you to connect screens between computers.

I think that's what I meant. 😁

Someone else mentioned it in another post. Someone with oodles of points so I believed them. 😉