In real life, most actions have an arched trajectory. To achieve greater realism, animators should follow this principle. Whether you’re creating the effect of limbs moving or an object thrown into the air, movements that follow natural arcs will create fluidity and avoid unnatural, erratic animation.
To keep arcs in mind, traditional animators often draw them lightly on paper to use as reference and to erase when they’re no longer needed. Speed and timing are important with arcs, as sometimes they happen so quickly that they blur to the point they’re unrecognisable.
Of course, this is sometimes done deliberately, to give the impression of something unrealistically or amusingly fast. This is known as an animation smear. Chuck Jones, one of the greatest animators of the 20th century, was an expert at these. He was behind one of the first examples in a short for Warner Bros in 1942. Jones only used it to save time, but liked it and would return to the trope for many animations in the Looney Tunes series. It's still used today in The Simpsons.