A soto uke is a karate blocking technique used for blocking mid-level incoming attacks. It is translated as "From outside block" like in O Soto Gari (leg throw from the outside), which comes from the block's final resting point centered on the blocker's body.
Soto uke is performed by bringing one hand to the ear, keeping the elbow raised, and sweeping the arm down and out towards the center of the body. The preparatory step of raising the arm gives the block more momentum, making it somewhat painful to encounter. A different application involves extending the non-blocking hand in front before performing the block. This represents grabbing the attacker's arm. The motion termed the block, then, is actually meant to break the arm, which would explain the necessity of the extra momentum.
Uchi uke is From inside block