The 'Tunnel Concept' is something that Mobility WOD's Kelly Starrett often talks about, and the concept is simple.
If you enter a tunnel in a bad position, what ever happens in that tunnel, its unlikely you will come out the other side in a better position.
Applying this to movement and exercise, he is talking about getting your set up of a lift right, to ensure the best possibility that after you've performed the loading movement (the tunnel) you will come out the other side in a good position.
Coming out of the Tunnel is dependant on two things:
The way you enter the tunnel
If you think about a Snatch as a good example for this (as seen in this video from Kelly). If your set up is all over the place at the start of your Snatch, as you begin to lift that weight, due to the bad set up at the start you are less likely to land the snatch (exit the tunnel) in a good position, or complete the lift successfully.
Watch this video for more information on how to appropriately set up:
How you move in the tunnel
Another factor is what happens when you are in the tunnel. You may have a great set up to start the movement, but if you immobile and find it hard to successful switch from position to position (i.e extension to flexion, or internal rotation to external rotation) then you are going to struggle to complete a movement successfully.
Using the Snatch (squat) again, your shoulders go though a series of movement patterns to complete this movement. It goes from being externally rotated (ideally) in the set up, to a period of internal rotation in the 2nd pull, back to external rotation overhead. On top of this your hips need to go from being flexed, to extended (important) back to flexed again. All these changes are happening in 'the tunnel'.
If you lack range or the ability to fully express these positions, you are going to struggle with completing movements.
The key to fixing this, is to mobilise :) and train your body in these positions.