A/C Orifice Tubes. The orifice tube acts as a small filter and restrictor inside the a/c system.
The Freon gas passes through the small screen mesh of the tube and any debris (mostly metal shavings that come from normal wear and tear of the compressor) will get trapped, and the Freon gas will also be slowed down and restricted as it passes through the small opening of the orifice. This is obviously just layman’s language, there is a science to how an a/c system works, and what it takes to keep it working properly.
There are different orifice tubes with different size openings/restrictions, and the color of the orifice can differentiate their diameter. It is a good idea to replace the orifice tube when replacing the compressor or a major a/c component, but some manufacturers say otherwise. Talk to your mechanic and see what they suggest. We replace them in my shop.