The beam weapon construction display lists the statistics of the beam weapons that you have both the resources and tech level to build at that time. When you build a beam weapon it remains in storage at the starbase until you fit the beam weapon onto a hull.
Beam weapons are the primary weapon system on most starships. They fire beams of powerful energy at a target vessel. Beam weapons do less damage than torpedoes and have a shorter range, but unlike torpedoes you can recharge a beam weapon and fire it again and again. When a ship fires all its torpedoes it must acquire more.
The display shows the cost to build this weapon in MegaCredits and the amount of material needed alongside a list of the total materials at the planet.