Your Mac is on the monitor
Use this screen as a trackpad: slide to move, tap to click,
two fingers to scroll, touch and hold then slide to drag.
Your keyboard and mouse control the Mac: this screen stands for the Mac's whole screen.
Three-finger tap (or Ctrl+Option+M) for the menu.
The Mac has no screen to show right now.
If the lid is closed, press Ctrl + Option + M (or tap with three fingers) and turn on 2nd screen or Lid closed.
Connecting
MacRemote
Your Mac, on your iPad.
My Mac
Connecting

Make sure MacRemote is running on your Mac and both are on the same Wi-Fi. Once connected, press Ctrl + Option + M or tap with three fingers for the menu.

MacRemote

Enter the 6-digit PIN shown in the Terminal window on your Mac.