Basically. When an HTTP request is made. There's usually a client and a server. Let's for example say you enter Google.com in your browser, your browser is the client because its the one requesting for the webpage that exists at google.com and google.com is hosted on a server (not one though) which accepts requests sent from clients like your browser and then returns the appropriate response.
Any programming stuff that take place in your browser in this case is called client side programming and any programming stuff that takes place on servers like the server that hosts google.com is known as server side programming