Until it is deployed, software is just files. Deployment is the act of putting those files onto servers so the website or app actually runs at a real address, for real users. "Deploy" is the verb, "a deployment" is one specific release going live.
Modern deployment is remarkably automated. A typical setup: the code lives on GitHub, a hosting service like Vercel watches it, and every approved change is built and pushed live automatically, usually in about a minute. Teams deploy many times a day this way.
Why you keep hearing it
Because it is the moment of truth in every building story ("we deployed on Friday", "the deploy broke something"), and because AI has pulled non-technical people into it. If you build something with AI tools, you eventually hit the question every developer knows: it works on my machine, now how do I get it on the internet? That is deployment.
What it means for you
Mostly, that shipping is no longer a ceremony. Fixing a typo on your website can go live in under two minutes, which changes how you think about iteration: publish, learn, adjust, again. If your team still treats every website change like a quarterly event, the bottleneck is process, not technology.