How the /effect Command Works
The basic syntax is /effect give <target> <effect> [seconds] [amplifier] [hideParticles]. The amplifier is zero-based: 0 is level I, 1 is level II, 4 is level V. Remove an effect with /effect clear <target> <effect>, or all effects with /effect clear <target>.
Examples
Speed II for 60 seconds on the nearest player.
Strength II with hidden particles, infinite, for everyone (infinite needs 1.19.4+).
Browse every effect on the Minecraft Effect ID list, or pair effects with potions from the Potion ID page.