Sourcing is the active side of recruiting. Instead of posting a role and waiting for applications, you go and find people who fit it: searching your own database, professional networks and past applicants, then reaching out directly. It is how you fill roles where the best people are not actively looking and would never see your job ad.
The strongest source is usually closer than teams expect. Your existing talent pool, the people who already applied, were referred, or were uploaded at some point, is full of candidates who have shown interest in your company and whose details you already hold. Reaching back out to a strong past applicant converts far better than a cold message to a stranger, because the relationship, however light, already exists.
Done well, sourcing turns hiring from reactive to deliberate. Rather than hoping the right person happens to apply this week, you build a pipeline of people you have identified as good fits and stay in touch until the timing is right. It takes more effort up front than posting a job, and for hard-to-fill or senior roles it is often the only thing that works.