Leads that come TO you (via content, ads, referrals). The customer initiates.
Inbound is the opposite of outbound. Someone discovers you via content, search, social, a referral, an ad, and raises their hand: demo request, trial signup, pricing inquiry. They're self-qualified (they came to you; they have a real interest). Inbound leads typically close at higher rates than outbound but take longer to generate.
Inbound scales at a different shape than outbound. It's slower to start (content takes time to rank, brand takes time to build) but cheaper and more sustainable once working. Best companies have both: outbound for pipeline now, inbound for efficient growth over time.