Instrument Flight Rules, commonly called IFR, are a set of rules that apply to planes flying by instrument reference. This is in opposition to VFR (Visual Flight Rules), flown by visual references. Basically, if you can see where you are going, then VFR is an option, but IFR is always an option.
This set of rules requires a pilot to have an instrument flight plan and follow a set of procedures that govern communication and navigation.
How do I know if I’m flying IFR?
If you have to ask, you’re not flying IFR!
But seriously, to fly IFR you will need to file an IFR flight plan, get a clearance from air traffic control, and comply with that clearance. You can fly IFR in VMC, but you can only log IFR, or “flight by reference to instruments” time when you are flying in actual IMC.