The process of germinating seeds indoors (or in protected conditions) under controlled temperature, light, and moisture, producing seedlings for transplanting after hardening off.
Start seeds indoors in seed-starting mix under lights 6-8 weeks before your last frost date. Keep soil warm (70-75°F for most crops) and moist. Once sprouted, provide bright light (grow lights or sunny window). Thin crowded seedlings. About 2 weeks before transplanting, harden off seedlings (gradually acclimate to outdoor conditions). Then transplant into garden.
Seed starting lets you grow a wide variety of plants cheaply. It's the foundation of vegetable gardening and cheap ornamental production.