These things prevent your eyewear from slipping down your nose.

They're called Keepons*.

They're sold in pairs.

To use them, you put them on your eyeglass temples like you'd put socks on your feet.

Then when you put your glasses on, the Keepons gently hook over your ears.

At the same time, they softly hug the sides of your head.

These two product behaviors thus stop the forwardly downward movement of your glasses, movement which is commonly referred to as eyeglass slipping.

*Fun fact - the word "Keepons" has no singular form. It's one of the few words in modern English that only exists in plural form. Other examples include: scissors, pants, trousers, glasses, pliers, tongs, tweezers, etc. Such words belong to the class of nouns known as Plurale Tantum.