The EM base-emitting frequency was an engineering term for measuring a phaser beam's component electromagnetic wavelengths.
In 2366, in preparation for a battle against a Borg cube, Geordi La Forge re-tuned the USS Enterprise-D's phasers to higher EM base-emitting frequencies. La Forge hoped this adjustment would disrupt the enemy ship's subspace field. In the battle, the new phaser frequencies had no impact on the field. (TNG: "The Best of Both Worlds")