What is Backlog
The ordered list of work a team intends to do but has not started, ranked by priority.
In plain terms
Everything a team might build waits in the backlog until it is pulled into a sprint. Items near the top are refined and imminent. Items near the bottom may never happen. 'It's in the backlog' often works as a polite deferral.
What it tells you about a developer
Backlog fluency signals product-minded engineering: the developer thinks about what is worth building next, which is the shape of a future tech lead. Someone with this fluency can usually walk through how their team decided what came off the backlog and why.