Cold outreach is contacting an investor with no prior relationship and no introduction from someone they already trust. It's usually an email or a LinkedIn message sent directly to a partner, associate or angel, often found through a fund's website, a portfolio list or a public database, and it usually needs to include a short teaser or a pitch deck so the investor has enough to judge quickly.
Investors get far more cold outreach than they can read closely, so a cold message has to do all the convincing on its own. There's no trusted person vouching for the founder or the company, which is the core difference between cold outreach and a warm introduction: a warm intro borrows credibility from whoever made it, while cold outreach has none to borrow. For an investor, cold outreach is still a source of deal flow, just usually a smaller and less trusted one than their own network produces.
Founders typically send cold outreach in batches once a fundraising round is open, rather than relying on it as their only route to investors. It tends to work best alongside warm introductions, not instead of them.