system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is an internet "route leak"?
gpt
This is greatly oversimplified but...

Routing on the Internet works by one operator telling its connected peers what networks they serve.  So, for example, if Verizon is ABC Corporation's ISP, they tell Cogent, AT&T, Comcast and everyone else they connect to that they can route to ABC's IP addresses.  The other ISPs use this information to determine where data gets sent.

A route leak happens when an ISP says it can route to a network they can't, either by accident or maliciously.  Sometimes customers have multiple ISPs and have to run the same routing protocols as the ISPs do for everything to work.  If a customer of, say, Cogent leaks a route into Cogent that they don't actually have and the route is attractive enough, traffic will be sent there even though Cogent has no way of delivering it.

A real world analogy would be putting up an "Exit here for Disneyland" sign along Interstate 40 in Oklahoma.  Traffic from the east coast would take that exit and end up nowhere near the right place.