The ideal is to get the clubface to the ball square at impact. Picture a line extending straight forward from the ball on the target line. Now picture a line extending along and out from the clubface (from the heel to the toe and out). If the club is square at impact, those two lines will be perpendicular, forming a right angle.
However, with an open face (a k a, a club that is open at impact), the toe of the club is turned a little rearward. The toe of the club is a little behind the heel at impact. So the angle that is 90 degrees when the clubface is square is more than 90 degrees with an open face.
An open face is one of the common causes of the slice and the push.

