What is this part of a double hung window sash called?
Those are safety catches that prevent the window from being opened all the way.

They pop out, and with the window unlocked, they let it open just a few inches. Supposedly an intruder cannot open the window enough to crawl in, and more importantly a child cannot crawl or fall out.

If these little catches are not in place then all you have are cosmetic closure caps.

Here is a photo: