Sharding, the art of scalability. That’s a bold statement, after all, there are two very important arts to scalability. What shards allow you to do however, is to scale out. What this means is, you no longer need to keep all of your data on one hard drive or machine, heck not even in the same warehouse or continent. You can have unlimited machines in unlimited geographic locations, connected together and serving data as if it was all the data was stored in one central location.
If you’re not already familiar with the terms scaling up/vertically and scaling out/horizontally, carry on reading. Otherwise you can skip the next few paragraphs.