Pretty much what it sounds like.  Can the player hold his own in one-on-one situations?  Later categories divide this up into the relevant problem situations, but you want an overall idea of whether this is a player who needs help on the line, or needs to be in a scheme that limits his isolation, such as a zone-blocking scheme.

Look for a flat back and pads lower than the defender.  Should drive off the ball and keep good leverage by staying low and keeping his balance.  Should bend easily at the knees.

Does the player get off the snap quickly and square up well with the defender? Can he "assist" the defender in taking himself out of the play?  The player should keep his feet driving, and should keep his toes forward, to prevent losing his base.  Should get good extension with his arms, and should "uncoil" well from the hips during the second phase of the block.

How quickly does the player get into the block?  Does he take good angles?  When the defender has the "angle advantage" does the player effectively redirect him?  Does he maintain his balance throughout the block or does he overextend himself?

How well does he communicate with linemates?  Is he in tandem with the line or does he stand out for being too slow, too quick?  Can he maintain his balance throughout the block, and does he keep his feet clean and moving?

How well does he drop and redirect?  His feet should be quick and his movements economical.  Watch out for many tiny steps, or leverage-killing long steps.  Does he step over trash well?  He should quickly be able to locate and engage his block in the running lane when he gets there.  Should have the ability to re-anchor at the point of attack and turn the defender.

Is his timing in tandem with the rest of the O-Line, and does he stay small in a tight area?  Should show good athleticism to operate fluidly in limited room.

How quickly does the player get to the second level?  Does he keep his balance and make contact with the LBs?  How quickly and efficiently does he find a block worth making?

After keeping his back flat, nothing is as significant for an O-Lineman as sustaining his block.  The player should continue to move his feet, and not try to stand up the lineman with his back.  Does the player let up before the whistle?  Better to see a player block too long than not long enough.

Specifically look for the player to maintain good position against the defender, and to maintain correct spacing with his linemates.  Should maintain his balance and remain square to his block.

A measure of the quickness of the player's feet.  For tackles their "kick-step" should be fast and clean, with a fluid, wide range.  Toes should stay pointing forward and not flare out.

Look for good quickness and the ability to stay inside on the chest of the defender. Should be able to re-establish contact if hands are batted away, and needs to maintain separation between himself and the defender.  (Note that some blocking techniques require the use of the forearms for additional blocking surface, but that the hips should still be capable of staying away from the defender.)

Useful for striking the defender and sapping the defender's momentum on the pass rush.

A measure of brute strength, can the player sit down in pass defense and hold ground against a pass rush.  Excellent quality for Guards and Centers working in tight spaces.

A combination of lateral agility and footwork, this is a measure of the player's ability to mirror a defender when shiftng the pocket.  The player should be able to work well in teandem.

When beaten by power or speed moves, how quickly does the player regain his balance and regain contact with the defender?  Watch out for players who are one and done when they get beaten.

This is the ability to control a defender and redirect him.  Usually the lineman will get his head on the defender's shoulder in the direction that he wants him to go.   Watch for this sign and note whether the player can effectively redirect to the area of his choosing.

As with run blocking, how long does the player stick with his block?  Anything less than to the whistle is cause for concern.

Has he?  Can he?  Just something that is good to know.  If he does have potential in this area, note whether he is a one-handed snapper or two-, and note the velocity and accuracy of the snap.