Here are some scenarios. You just saw some cool cichlids online and you're going to order them. Maybe you just saw a few species of awesome new cichlids at your LFS and plan to bring some home. Perhaps you just got a pay increase or you're expecting a tax refund this year. The wheels in your head are spinning because you want that new tank, you know what cichlids you're going to put in it, and you now have the funds to make it all happen. So you do. Now the tank is at home and you're already planning to fill it up and get the fish.
Oh, hang on a second. You just remembered that you have to cycle that tank. Whether you're planning to cycle it the traditional way (with a few hardy fish) or whether you're going to do a fish-less cycle by dosing with ammonia, you still have to wait.
You've read about it or heard someone mention it but you've been afraid to ask what it means. What is seeded media or, more accurately, seed media? It's simply bio-media from a stable tank that is saturated with bacteria, which can be used to begin breaking down ammonia and nitrite immediately in an uncycled tank. However, don't confuse "immediately" with "sufficiently." Those aren't the same thing.
The amount of bio-media you'll need to handle a new bio-load varies. Unless you're a scientist, that will have to come with experience. A small amount of bio-media will typically only kick-start the cycling process. However, a large amount may well be enough to prevent any stage of the cycle from restarting, which means the new tank can be ready for fish immediately.
Sure, you can buy some of the "cycling" products on the market to get things going, but using seed media is the best way to go. So where do you get it? Get some from a fellow aquarist, ask for some from your LFS, or keep some at the ready yourself. That last option is what I do, and it's easy.
The photo at top is a media bag that contains two types of bio-media. This is one of two bags of extra bio-media that I keep in the corner overflow of my 75g tank all of the time to serve as seed media. You can do the same thing in a sump. If you don't have an overflow or sump, you can always "rob" your canister, your HOB, or other filter of a portion of bio-media. However, be careful with the "robbing" plan. Removing too much media from an existing filter can diminish the capacity of that filter to manage the bio-load of the tank it's filtering.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub