As detailed online, these are both two different drawing techniques in the animation process.
Straight ahead entails goign frame by frame in animation and drawing with no set point to work your animation towards – for example, you’re moving a character from point A to point B, and to get there you’ll draw hundreds of frames in order from A to B.
Pose to Pose involves drawing the key points in between A and B, marking perhaps a qaurter way and half way point. After these have been filled in, the individuals frames between these key points can be filled in.
They each have their own benefits but Straight Ahead is better for more fluid action that maintains Dynamics. Pose to Pose on the other hand is better for scenes where proportions is of more importance as the method to draw in Pose to Pose will ensure accuracy.
Both Techniques can be used together.
In the image above, Pose to Pose would be demonstrated by using the key frames above and filling in the missing frames in between.
Straight Ahead is demonstrated above with each frame comign straight after the other in order.