New User - edit mode frustration

I've just installed PyCharm on an iMac.

I'm new to PyCharm and Python, but not development.

I'm struggling with editing modes that seem to keep switching: one minute I have a normal cursor and I can edit text, and seemingly without warning the cursor changes to a white square and I can't type anything, all I can do is move the cursor around.

Can anyone tell me now I turn this off, or otherwise fix this, as at the moment PyCharm is unusable for me.

Thanks