Beam has several related meanings:

1. A long, thick piece of wood or metal used to support the weight of a building or structure. Ceiling beams hold up the floor above you. The main beam in a house might run from one end to the other, carrying tons of weight. Ancient temples used massive stone beams to span the space between columns. When builders put up a new building, securing the steel beams is one of the most critical steps.
2. A ray or shaft of light. Sunlight streaming through a window comes in beams. A flashlight sends out a beam of light into the darkness. Lighthouse beams sweep across the ocean to warn ships away from dangerous rocks. You might see beams of sunlight breaking through clouds after a storm.
3. To smile very widely and happily. When you beam with pride, your face lights up with joy. A parent might beam at their child's piano recital. Someone who just won an award might be beaming from ear to ear. The word suggests a smile so big and bright that it's almost like light radiating from your face.

The balance beam in gymnastics gets its name from the first meaning: it's a narrow beam (just four inches wide) that gymnasts perform routines on, testing their balance and courage.