Plain English Meaning
Before flying on instruments, the pilot must confirm that all the navigation gear they will rely on is working and has been formally checked recently enough to be legal and trustworthy.
Full Definition
A pre-flight verification that the navigation instruments needed for the intended IFR flight are operational, properly indicating, and have been checked within the timeframes required by regulation. This includes confirming that VOR receivers have been checked within the preceding 30 days for IFR use, that the altimeter and static system have been tested within the preceding 24 calendar months, that the transponder has been tested within the preceding 24 calendar months, and that the pitot-static system, attitude indicator, heading indicator, and other required instruments are functioning correctly before .