A character’s Speed is how fast they can move, which, unless otherwise noted, is 6. Often play is done on a grid with 1 inch squares or hexagons representing roughly 5 ft. or 1 meter each. So a hero with a speed of 6 can travel up to 6 spaces horizontally or diagonally.
You can move through spaces occupied by allies (or enemies as difficult terrain: half speed), as long as you don’t end movement in an occupied space.
Alternative Options For a quicker, more loose game, you can forego counting spaces and measure typical movement roughly from pinkie to thumb. Slightly less for players with large hands (or slower characters), a bit more for our tiny-handed friends (or faster characters).