Animation - Principles 4 - 6: Straight-Ahead and Pose-To-Pose, Follow-Through and Overlapping and Slow In/Out

Straight Ahead and Pose to Pose: Both techniques used likely in unison, straight ahead and pose to pose is the approach an animator takes to constructing an animated movement.
Straight ahead is drawing frames one after the other, having a rough idea of what happens next but without a frame in front to guide the current. this can cause scaling issues as one usually isn't aware of how inconsistent their animation is until they eventually play it through.
Pose to pose is drawing out each key motion first then filling in the frames in between creating a more controlled animation. Pose to pose isn't as good at producing animation that's suppose to be unique and natural, instead of creating a foundation that could be added to using straight ahead. For example, you could create a jumping person using pose to pose and then add secondary features such as hair, loose clothing or other accessories afterward using straight-ahead.

Follow through and overlapping: These principles give a movement more realism by implementing the laws of physics. When one moves their arm, it doesn't just stop but sway depending on how fast the initial motion was. The intensity of this "sway" can convey a lot about the weight and mass of the object, similar to squash and stretch.
Similar to how it's important to add a few frames beforehand to anticipate a motion, it's equally as important to follow through with a few frames of sway.

Slow-In and Slow-Out: This principle focuses on the rate of an animation. If an animation is constant with any easing, it can make movements seem fake and automated. Starting and ending a movement slow before speeding up creates more believable, lifelike motion.
Having a character dramatically move from being still to fast isn't an exaggeration, it's the signs of an impatient animator. Even if it's just two or three frames, it improves an otherwise unrealistic movement.
It's also important to consider the spacing between frames being too inconsistent and resulting in choppier animation. Frames should be evenly spaced and aligned which is usually done toward the back end of an animation once all frames have been drawn.