Unless you've been highly disconnected or working heads down for the last 5 years, you've probably heard of NoSQL databases. You might have had developers ask to use one. Even if you might never have actually dealt with one, sticking inside your SQL Server comfort zone, you've probably heard of them.
Recently I was watching someone talk about CosmosDB, which I think is a great platform. I've actually be looking for a project to use and experiment with the platform, and I think Microsoft has truly built an attractive platform that might get lots of people to consider Azure for distributed applications. CosmosDB essentially implements a number of alternative database APIs, many of which are thought of as NoSQL platforms.
So, what does NoSQL mean? Plenty of people view this as No SQL Allowed, but that's not really correct. This moniker was used to mean "Not only SQL", with the intention that there are perhaps other ways to store and query data. Graph databases and document databases seem to be the most common ones that are discussed. Key-value stores and in-memory databases, such as Redis, are other very popular data stores.
There are plenty of other choices for a place to keep your data. If you examine the rankings of database engines, you will find over a 100 different databases in use. The top ten are mostly relational stores, as are most of the top 20. There are plenty of other types in the list, and NoSQL databases are certainly gaining popularity. While I don't think they will replace the use of an RDBMS for many applications, I'd expect that we will see more applications built with backing from multiple data stores.
There are very useful NoSQL databases out there, and you might consider spending some of your learning time working with one. If you're in the Microsoft space, CosmosDB makes the most sense, and this will allow you the opportunity to work with a few different types of NoSQL APIs. If you're worried about costs, you can use the local emulator on your own system for free.
I don't anticipate NoSQL taking over a majority of my job anytime soon, but I do think it's worth understanding how the different stores work and where they might help you solve a problem. Especially if you're optimistic and believe your application might scale up dramatically in the future.