Let's begin by defining what software architecture actually is. When you create an application, library, or any software component, you need to think about how the elements you write will look and how they will interact with each other. In other words, you're designing them and their relations with their surroundings. Just like with urban architecture, it's important to think about the bigger picture to not end up in a haphazard state. On a small scale, every single building looks okay, but they don't combine into a sensible bigger picture – they just don't fit together well. This is what's called accidental architecture and it is one of the outcomes you want to avoid. However, keep in mind that whether you're putting your thoughts into it or not, when writing software you are creating an architecture.
So, what exactly should you be creating if you want to mindfully define the architecture of your solution? The Software Engineering Institute has this to say:
This means that in order to define an architecture thoroughly, we should think about it from a few perspectives instead of just hopping into writing code.