Cookies are small files that the browser uses to record data about sites you've visited. Usually sites save cookies in the user's browser in order to easily "recognize" the user (for example, to avoid asking for their username and password again after the user recently visited a given site).
Using cookies has some privacy risks. For example, if your passwords are saved in cookies and set to autofill, an unauthorized person who got hold of your smartphone could sign into your accounts on various websites. Someone can use your cookies to find out what pages you've visited or discover correlations between various pages you've viewed.
To protect your privacy, or just to free up some space on your phone smartphone, you can delete all your cookies.