Different looms work a little differently, but here is the basic idea…
The warp threads run from a back beam (warp beam) which is a bar the warp is tied to at the back of the loom, to a front beam (cloth beam) at the front.
A the start of your weaving, most of the warp is wound around the back beam and, as you weave you, roll the back and front beams forward, so the cloth you’re creating wraps around the front beam.
On the way from back to front, each warp thread passes through a heddle and a reed.
The heddles are attached to different shafts/harnesses and these are attached (usually tied) to some type of lever.
You move a lever with either hand or foot and this lifts a shaft/harness, pulling the attached heddles upward. This lifts the warp thread that runs through those particular heddles and creates the “shed” (sideways-V shaped gap) for you to pass your weft thread through.
Each lever will lift a different shaft/harness and so a different group of warp threads. The more shafts/harnesses you have, the more complex the patterns you can do because you have more variations in lifting different groups of warp threads.
My rigid heddle loom does the same thing, but without the levers, as the reed – with its built-in heddles – acts as two shafts in one. You manually raise or lower the reed to create different sheds.
Don’t worry if that’s not 100% clear. The rigid heddle will get a more detailed post of its own later! In the meantime, YouTube is a great place to find video of different looms in action.