Prediction Error
Basically, the servers distribute information about all players and their actions to all connected players of a match. But it takes a while for the data to reach each player, so in the meantime, the client machines "predict" what each of those other players is going to do. if [enemy] is running in a straight line, it's very likely that they will keep running in a straight line, so your machine can animate [enemy] running even if it doesn't know where [enemy] is.
However, if the server has difficulty providing enough updates at high enough speed, your machine will be "starved" of information and realize it is very likely going to draw enemies in wrong positions. For instance, if the server cannot push packets every 50 ms for whatever reason, but only every 150 ms, that's a lot of time during which enemies are routinely going to change direction or do something unexpected. This makes it harder to hit enemies, because you are firing at "predicted" positions, but your shot will be evaluated by the server against their actual position.
The symbol for Prediction Error is 2 dotted lines (3 dots on each), with the second line being shifted against the first, illustrative of how the game displays an enemy at different position from where they actually are (think footsteps in snow).
Packet Loss
Server distributes information about players' actions in packets. Each packet is numbered. Your machine receives these packets, but they are distributed using an internet protocol that doesn't require your machine to confirm receiving each packet, nor does that protocol re-send packets that your machine never received. So, if your machine notices that it received packets 37, 38, 42, 43, 44, 48, 49... it will realize that several packets were lost, and will indicate such using the packet loss symbol (three squares, with the middle one being "partially missing")
Correction edit: Replaced original numerical values with ones which are more appropriate for Apex Legends.