Imagine that you have a sink with a very narrow drain. Your friend pours a large bucket of water into it. The sink is full and draining very slowly.
You have a spoonful of oil that you want to get down the drain as soon as possible. You pour it into the sink. But of course, it doesn’t drain quickly - the water in the sink needs to drain first. Shoot!
The sink is a buffer - a “queue” for liquid. When your friend poured in the bucket of water, they filled the queue. Anything new will take a long time to drain.
This isn’t a perfect analogy to what’s happening in your router, but it’s close!
Instead of water and oil, networks have different flows of packets. Your router is like the sink, and your connection to the ISP is the narrow drain (since it’s probably the slowest link in your network).
When someone on your network sends a large file, a lot of packets get sent all at once. The router temporarily "buffers those packets", holding them before they’re sent. Any new data packets get stuck behind the existing queue of buffered packets. They will arrive at the destination much later than if the router’s buffers hadn't been full.
This is “bufferbloat” - undesirable high latency caused by other traffic on your network. It happens when a flow uses more than its fair share of the bottleneck. Bufferbloat is the primary cause of bad performance for real-time Internet applications like VoIP calls, video games, and videoconferencing.
However, all is not lost!
Certain routers have smart algorithms (usually called "SQM") that ensure that time-sensitive packets flowing through the router don’t get delayed, even when large files are being downloaded or uploaded. Continuing with the liquid analogy, these routers offer a way to admit just the right amount of water into the sink so the drain pipe is always full, but a new spoonful of oil will drain out immediately.
