Original: The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back.
Simplified: Bilateria are all animals having a bilaterally symmetrical animals, they have front and a back end, as well as an upside and downside.