@Ben's answer is correct. I'd like to explain how packet loss occurs and how it affects you.
When two computers communicate, they essentially send "packets" to each other. Sometimes, a packet never reaches its intended recipient. That's a "packet loss".
Packet losses can happen when the line between both computers is damaged, or when any of the devices between the two is overloaded, such as when there is too much traffic and the router has to ignore a packet or two.
How does it affect me?
If you're streaming, packet loss can manifest in a loss of quality. Missing packets means missing information, which can cause frame skips, audio skips, or glitches in the video.
Not all streaming services are affected by the above loss of quality. If you experience packet loss on YouTube, it's much more likely that your browser will say "I didn't get that package", and the server will simply resend the packet. Which leads us to the next point:
Packet loss usually affects connection speeds. Ideally, your PC sends a packet, and receives a packet in return. When a packet is lost, it has to be resent. Imagine if two people are talking, and one asks: "what did you say?" Obviously, the other party has to repeat himself. As long as it doesn't happen often, all is well. If it does happen often, however, the communication speed could slow to a crawl.
Packet loss is especially bad if your ping is high. A high ping means it takes longer for your packet to reach the intended destination, and for a response packet to come back to you. If you have a ping of 1000 milliseconds (1 second), each packet loss means you have to wait 1 additional second for the response packet to arrive. With a ping of 100 milliseconds, you could afford 10 packet losses in a row for the same effect.
In a game like Fortnite, a packet loss usually causes lags due to a sudden increase in ping. Even if the game doesn't request for the packet to be resent, it still has to send a new packet and wait for a new response.