After the warp threads have been stretched on the frame, the heddles have to be set. The heddles are the part of the loom, typically made of wire, that separate the warp threads. The heddles – “batei nirin” in Hebrew – raise the warp threads successively, enabling the woof threads to pass over and under them. Setting the heddles is the melacha of oseh batei nirin.
Now our loom is all ready to go. In the next melacha, we come to the actual weaving.
This is just an introduction to the concepts of the melacha of oseh batei nirin; it is not a substitute for a full study of the halachos.