docs: small improvement to first customization guide (#9843)

Added a bit more context and what you'll build in the first customization guide and why it's powerful.

Closes DX-1044

www/apps/book/app/learn/first-customizations/page.mdx

@@ -6,6 +6,10 @@ export const metadata = {
 
 In this chapter, you’ll build your first customizations with Medusa.
 
+You'll create a `GET` API route at the path `/hello-world`. An API route exposes your application's commerce features to frontend clients, such as the admin dashboard or a storefront.
+
+Medusa has [Store](!api!/store) and [Admin](!api!/admin) API routes, and you can create custom API routes for your custom features.
+
 ## 1. Create API Route
 
 API Routes expose business logic through REST APIs. You can create custom API routes to expose custom business logic.