"Cherokee five mike charlie, turn left heading 120, intercept the runway 8 Left localizer." Most instrument pilots have received an ATC instruction like this, but many don't understand its meaning. Implicit in this instruction is a limit not to descend or otherwise fly the approach procedure via your own navigation. Some pilots confuse this instruction with an approach clearance and believe they're cleared to descend and fly the charted procedure. Not yet. That clearance will most likely come shortly.
When a controller issues an instruction to "intercept the localizer," he's giving you a simple lateral instruction, much like an instruction to join an airway. He does not expect you to do anything other than just what he said, join the localizer. You might also hear a controller use this phraseology in lieu of "intercept." "Join the localizer" carries with it the same meaning and expectations. Turn onto the localizer, fly it, and await further clearance. DO NOT fly the approach procedure until you hear the words "cleared for XXX approach." You're not authorized to descend or make any turns called for by the approach until you've received approach clearance.
When a controller does issue an approach clearance, he's releasing you from his instructions to fly the approach under your own navigation following the charted procedure's courses and altitudes. Until you hear those magic words, "cleared for XXX approach," the controller is keeping you tight on his leash and you must obey only his commands.
So, why do controllers issue instructions only to intercept the localizer instead of fly the approach? Controllers have rules about when they can issue approach clearances to aircraft depending on traffic and your distance and altitude from the airport or some other fix. The controller can't let you off his leash to conduct the approach procedure until certain conditions are met. He'll clear you when he can, and in the meantime maintain your assigned altitude and simply do as he says, intercept the localizer and track it inbound.
