When a logo has multiple elements, such as an icon and a wordmark, a lockup is the exact arrangement of those elements or the relationship between them in a logo.
Here's an analogy.
Think about the iconic Amazon logo. It includes multiple elements, including the wordmark that displays the company name and a yellow arrow pointing from “a” to “z.” These elements work together to form a lockup.
Why does it matter?
Logos require a lot of thought from your product brand and product marketing teams. When brand designers develop a logo, they have to consider its many components and what they communicate. When you add new elements to your logo, it can change the meaning.
How does it apply?
Logos usually go through many iterations, and they can change as your brand evolves. Lockups are one of several ways to design your preferred logo and communicate what you want.