What is Packet Loss?
You may be asking, "What is packet loss?" or "Why should I care?" Well, packet loss (also known as "packet drop") is simply when a packet is not able to be delivered. It's sent, maybe even received by the server, but somewhere it gets lost. It's like, if you send some message to a server, but the server never receives it or the other way around (It sends it and it fails to reach you.). Regardless of the direction, this is bad.
With typical, TCP-based web browsing, the sender will realize that its message was not delivered properly and will resend it until it goes through right or gives up, but this still adds significant slowdowns because it has to wait long enough to give up on it being delivered before starting the whole delivery process over again.
With UDP and similar unreliable protocols, messages will usually not be attempted to be redelivered and are just lost for good. In a voice call, this might mean some words are just never sent. In an online game, this will usually mean that something you tell the game to do never happens. Your movement gets undone or your bullets never register. This can understandably be very frustrating, and any significant amount should be attempted to be fixed, but to do that, one must be able to test if it is present.
See also: What Causes Packet Loss?