Every AI product quietly hands its model a set of instructions before your conversation starts: what role to play, what tone to use, what it may and may not do, what it knows about the current situation. That hidden briefing is the system prompt. Your messages arrive on top of it.
This is a big part of why the same model feels different in different products. A customer service bot, a coding assistant and a writing tool can all run on identical technology, separated mainly by their system prompts and their harness.
Why you keep hearing it
Because system prompts are where a product's personality and rules live, and they occasionally leak, which makes for entertaining reading. Also because building with AI, even lightly (a custom GPT, a Claude Project), means writing one yourself: the instructions that make the assistant yours.
What it means for you
If you set up AI assistants for your team, this is the layer where brand voice, standards and boundaries belong. Write down who the assistant is, what good output looks like, and what it should never do. A well-written system prompt is the difference between an assistant that sounds like your brand and one that sounds like everyone's.