A custom GPT is a feature of ChatGPT that lets you create a specialised assistant without writing any code. You give it standing instructions (who it is, how it should sound, what rules it follows), optionally upload reference documents, and save it. From then on, it behaves that way in every conversation, and you can share it with your team or the world.
Under the hood it is mostly a saved system prompt plus your documents. That sounds unglamorous, but the effect is real: a content assistant that knows your brand voice, or an analyst that knows your reporting format, beats a blank chat window every time. Claude's equivalent is called a Project, and most AI platforms now have some version of the idea.
Why you keep hearing it
Because custom GPTs are the standard first step from using AI to building with AI. No code, an afternoon of work, immediate usefulness. Workshops and courses (including mine) use them as the gateway exercise.
What it means for you
If you use the same instructions in a chat window more than twice a week, you are overdue to save them as a custom GPT or Project. Think in roles: a content person, a critic, a researcher. Build the colleague once instead of re-briefing a stranger every morning.