A Beam in Roblox is a visual effect that draws a line or textured strip between two Attachments. In practice, people use it for things like lasers, magic effects, electricity, guide lines, or force-field style visuals.

In Arachnidia

If you saw “beam” in Arachnidia, it most likely means that same Roblox Beam object or a beam-like attack/effect in the game’s build. Roblox’s Beam object renders a texture between two attachments and can be customized with color, transparency, width, and texture settings.

Simple example

If one attachment is on a spider leg and the other is on a target point, the Beam can visually connect them like a web strand or energy line. That’s why it often looks like an arrow, laser, or rope effect in-game.

TL;DR

A beam is basically a customizable visual connector between two points in Roblox.