What it means
An ideal customer profile (ICP) describes the type of company that gets the most value from what you sell and is most likely to buy it. It is written at company level: industry, size, location, the tools they run, how they are funded and the problem that makes them a fit.
A buyer persona describes a person inside that company, so the two work together. A good ICP is specific enough to exclude accounts. If it fits every company in your CRM, it needs another pass.
How we use it
For us the ICP is a file, kept under version control inside each client's GTM brain with the offer and the proof. Every skill reads it at run time instead of inventing one.
Accounts are scored against it, and we track the ICP as it converts, set against the ICP we started with. A pricing-page visit from a company outside the ICP is noise, however hot it looks, so every buying signal is checked against it first.