It’s a channel nut, with a spring on one side, that slides inside of a piece of strut channel. The spring takes up the extra space inside the channel, keeping the nut flush against the inturned edges of the channel, so you don’t need an extra hand to hold it in place, while you’re busy trying to affix parts and tighten them down. The length of the spring helps determine which channel profiles the spring nut can be used in: longer springs for deeper channels and shorter for shallow channel.