A blindside tackle in football is one in which the defensive player, unseen by the quarterback, makes contact with and tackles him, particularly on passing plays. The major concern in allowing a blindside tackle is that the quarterback will not see the hit coming and may be more prone to fumble the football.
Because most are right-handed, quarterbacks turn their bodies to the right in order to survey the fields and consequently throw the ball. This means that a blindside tackle will almost certainly come from the left side of the field, as perceived by the offense. Conversely, if the quarterback is left-handed, a blindside tackle will come from the right side.
Generally, the best offensive lineman on a team has is placed at the left tackle position. The idea behind this coaching decision is to keep the defense from getting past the offensive line and earning a sack by way of a blindside tackle. More recently, however, defenses have placed better pass rushers on the right side of the quarterback, deemphasizing the idea that a blindside tackle is much more important than visible pressure to the quarterback.