All communication across a network is broken into packets before sending and is reassembled at the receiving end. Suppose you are sending a 100 page letter to someone and their mailslot is only big enough for one page at a time. So we have to send each page in its own envelope with its own addressing and stamp. Some of the envelopes are delivered. Usually the latter case occurs.
Most systems measure success by how many of the envelopes are delivered.
Some protocols will try to resend lost envelopes. When packets are mashed or don't arrive at all, each web browsers will give slightly different error messages. Some are just downright misleading about what happened. Usually they will notify you that the server is down. Nope, not right.