what is the difference between dot slash and without dot slash in url relative paths?

in HTML when I ask for a resource maybe CSS or js

suppose my URL is xyz.com/contacts/index.html

if I say /a.js it looks at the root folder and request will go to xyz.com/a.js

if I want to request in contacts path I can do a.js or ./a.js

I wanted to know what is the difference between a.js and ./a.js?