Link prospecting is the research part of link building: finding posts and pages where your product genuinely belongs, then checking whether each one is worth an email.
Good prospects are relevant to what you do, rank or get read, are maintained, link out to tools like yours, and have a findable person to contact. Bad prospects are unrelated sites with high scores, abandoned pages and sites that sell links.
Most outreach that fails, fails here. A strong email to the wrong post rarely works; a plain email to the right post often does.
Example
Margin searches "best note-taking apps for researchers" and "Evernote alternatives for academics", keeps the 30 posts that list similar tools, and drops the rest.
Putting it into practice
Where to use it: Link prospecting search strings and How to choose sites to pitch. For the longer version, read finding every roundup post in your niche.