Traction is a measure of how long it takes for a character to stop moving from a sideways force when grounded. Characters with low traction slide farther than others after walking, dashing, wavedashing, being hit by an attack while grounded, and so on.
Having high traction affords greater ground control over a character, so high traction is generally an advantage. Characters with low traction generally require more precise movement across the ground and cannot punish attacks as effectively out of shield since they will get pushed further away from the opponent. However, there are cases where low traction is useful: low traction is necessary for a long wavedash in Super Smash Bros. Melee and a long dash pivot cancel in Brawl, and there are cases where sliding away from a shielded attack can help escape multiple hits. In Super Smash Bros., characters with lower traction get pushed further away when being attacked when in shield, which makes follow-ups and potential shield breaks harder.
Luigi is well-known for having the least traction of any character in all games except Super Smash Bros. Ultimate; this originates from his low traction in most of the Mario platformers, which balances out his higher jumping.
Traction is also affected by terrain. For instance, nearly all characters have decreased traction on slippery surfaces such as ice. However, the Ice Climbers uniquely have the reverse of this functionality: their normally low traction becomes higher on slippery surfaces.
In Ultimate, traction has been significantly increased across the cast, with even the lowest traction values being higher than the highest traction values in SSB4. This allows for every character to slow down faster on the ground after moving horizontally, and most notably benefits characters with low traction (such as Luigi and Charizard), as shielded attacks now push them back a much shorter distance, and it is therefore easier for them to punish out of shield. However, the increased traction also results in slide smashes covering less distance than in SSB4 as well. Unlike in previous games, Luigi no longer has the most slippery shoes of the cast, nor is he even in the bottom tenth; the characters with the least traction are now the Mii Gunner and Banjo & Kazooie.