Range, also known as reach, is a general term used to describe how far an attack's hitbox reaches relative to the attacker. Range is measured based on how far an attack extends from a character when in a stationary position. Attacks with disjointed hitboxes generally have more range, such as most of Marth's attacks, Simon's whip attacks, and Ike's aerials, allowing them to attack opponents from a safe distance. On the other hand, many grabs have poor range: a user must get in very close to be able to connect with them, which is usually risky to do. Projectiles in general have very long range.
Although various attacks have various ranges, certain characters tend to have longer reaches than others, allowing them to space more easily. For example, most characters with a weapon (sword, hammer, whip, etc.) have longer range than ones who do not. Characters with short range are forced to approach more often, while those with longer range can often afford to camp. Larger and heavier characters, such as Donkey Kong, tend to have better reach than smaller and lighter characters, such as Pichu, in exchange for a larger hurtbox.
There are attacks that are considered to have "long" range, when the character is merely rushing ahead to attack. An example of this is Wolf's forward smash in Brawl. Additionally, there is also variation in attack hitboxes. Falcon Kick, for instance, covers a lot of ground, but the range of the move is only as long as Captain Falcon's leg. Pikachu and Pichu's Skull Bash and Luigi's Green Missile cover more ground, but the hitbox on those moves covers the entirety of the attacker's body. Wario's Wario Bike is unique in that it covers ground as well, but is actually an item that attacks. Sonic's Spin Dash side special and Spin Charge down special are special: his body is the hitbox and he keeps going after connecting, but keeps momentum.
It is generally considered advantageous for a player to have a longer range than their opponent, as they can safely hit said opponent without the risk of being hit back. There is much more to this element of gameplay in terms of spacing, but longer ranged attacks being better holds true in the simplest terms. However, as a form of balance, characters and attacks with longer range tend to be on the slower end of the spectrum with high amounts of lag. This means that long range attacks tend to lose up close with characters that have shorter range, but are faster and can get more moves out.