Spacing is the utilization of a character's range in comparison to the opponent's range to ensure the character's attacks make contact and that the opponent's do not. Several factors contribute towards a character's ability to space. Characters with large attacks, like Donkey Kong and Ike, can rely on sheer range to space their opponents: Ike's forward aerial has massive horizontal and vertical range in front of him, making it one of the best spacing tools in the game. Disjointed hitboxes like swords can further improve a character's ability to space. Aerial speed and acceleration are also factors - characters like Wario and Jigglypuff can "fade in and out" with aerials using their amazing air speed, allowing them to compensate for their attacks' otherwise poor range.
Spacing is most commonly performed with aerials, but other attacks can be used to space as well. Tilts tend to be useful in this regard - Donkey Kong's and King Dedede's forward tilts are especially effective due to their incredible range. Certain special moves with long range, such as Zero Suit Samus's Plasma Whip and many grab aerials, can also be used to space, and many projectiles are especially useful as spacing tools due to their extreme range.
Spacing is a key component in the neutral game; without the proper tools for spacing, a character will have a hard time approaching, camping, or fighting against approaches or opposing camping. This can be somewhat mitigated depending on the stage, as platforms can drastically affect spacing: for example, Battlefield's top platform can force approaches from below, which often limits the opponent's spacing options as few projectiles can travel upwards.