From SmashWiki, the Super Smash Bros. wiki
A strong attack, also known as a Tilt Attack or simply a Tilt, is a ground attack performed by lightly pushing and holding, or "tilting", the control stick in a direction and pressing the attack button, but not smashing the control stick as for a Smash Attack. They typically have good range but less knockback than a smash. The easiest way to preform a strong attack is to either press the attack button while running/walking or crouching. Like Smash Attacks, there are Strong attacks to the side, up, and down. Some side tilts can be angled up or down (such as Captain Falcon's). These moves can damage the opponent well while also keeping them in range for more attack combos.