If I turn iMessage off, will I receive those messages when I turn it back on?
You need to turn off iMessage on all the devices that have it on. If you have iMessage enabled on iPhone, iPad, and Mac, turning one off will not disable iMessage entirely.

For example, if you enable iMessage on iPhone and Mac, but only turn off the one on iPhone, People can still send iMessage your phone number or email linked to your iMessage account on Mac.

Answering your question, the message will appear on the devices that has iMessage but not on the one you’ve disabled, however, if you turn off iMessage on all devices, then your friends will send you a normal text message.