Another take.
Belichick is talking about how to defend the an offensive package - specifically where to send the middle linebacker.
He is qualifying that with his opening comment "Even on the play-action stuff."
That means he is about to suggest the best way to run the middle linebacker, recognizing that sometimes in this offense the line will block as if it's a run, but will use a play-action to pull in the linebackers and then throw. The risk here is that the middle linebacker vacates his position, where a receiver (including tight end or running back) could be to receive the play-action pass.
"in 21"
As Tony2046 mentioned, 21 refers to an offensive personnel package. 2 RB, 1 TE. That leaves 2 WR.
A 21 is the standard personnel package of the 80's and 90's. Now three-receiver sets are more common.
Anyway, a 21 package has a tight end and a fullback, so it's a standard package for a run.
"getting the Mike"
Linebackers are typically referred to as Sam, Mike, and Will.
Sam is the strong-side linebacker, which means the side of the line with more offensive blockers. Commonly the extra blocker is a tight end, but could a fullback or extra offensive lineman. The Sam faces the most traffic, and is normally on the front side of the play.
Mike is the middle linebacker. The Mike typically gets the defensive call and aligns the rest of the defense.
Will is the weak-side linebacker, and can blitz or drop into coverage. The play often goes away from the Will, so the Will is chasing from the backside.
"in there on the front side"
This is the direction in which the ball is going. Guards are typically referred to as frontside and backside guards, based on the direction of the play. In a standard 21 package run behind the tight end, the tight end aligns to the right, outside the larger right tackle. The fullback will align to the right, and the running back will run to the right behind the overloaded blockers. What Belichick is saying, then, is to align the middle linebacker over to the right, and prepare to flow to the right at the snap. Get toward the front side of the play.
"against the slide"
Tony2046 presented a nice chart showing a simple slide blocking scheme. No guard pulling, just basic run blocking moving forward toward one side of the field.
"is a lot better than getting a read game"
Not positive here, but I am assuming he means putting the middle linebacker in the more likely spot at the start is better than waiting to diagnose, or read the play, once it starts to develop. If you stay in your normal position and read the play, you have a better chance of knowing where to move into exactly the right spot, but probably too late.
"on the backside where it's still 4 on 2"
So which player is normally on the backside? The Will linebacker. The Will is aligned to the backside of the play, protecting that side. If the Mike waits, reads the play, and heads toward the backside of the play with the Will, there will be two defenders. I think this means there will be four blockers for those two defenders. That would be some combination of full back, tight end, quarterback, and offensive linemen, depending upon the specific formation. Assume that the tight end picks up the strong-side DE, the right tackle and right guard the two defensive tackles, and the center picks up the weakside defensive end. The backside of the play then has the left tackle, left guard, and fullback, as well as the QB. The specific blocking scheme and pick-ups could differ, but you still have eight offensive players around the running back (5 OL, 1 TE, 1 FB, 1 QB), so the four defensive linemen can be picked up by four of them, potentially leaving four available blockers on the backside. Recall one of the occasional Patriots runs where the running back sees a wall of defenders to the right, then cuts back around the backside and you see Tom Brady sort of move out in front of him.
So he is suggesting that even if the Mike waits to read the play, he will still be at a disadvantage due to the number of blockers, so positioning the Mike to the frontside is the best plan.
