Bed adhesion is perhaps the single most critical factor in the success of a 3D print. It refers to the ability of the extruded plastic to stick firmly to the build plate during the duration of the print. When adhesion fails, the results range from minor warping at the corners to a catastrophic "spaghetti monster" where the part detaches entirely and the printer continues to extrude plastic into mid-air.
The most common cause of poor adhesion is an incorrect "Z-offset," which is the distance between the nozzle and the bed on the first layer. For the plastic to grip the surface, it needs to be slightly compressed or "squished" against the plate. Temperature management is also vital; most FDM plastics shrink as they cool, causing warping. A heated bed helps mitigate this by keeping the lower layers above the material's Glass Transition Temperature for longer.
Finally, surface cleanliness and preparation are vital. Microscopic amounts of grease from your fingertips can act as a release agent. Regular cleaning with high-purity Isopropyl Alcohol (IPA) is recommended. Additionally, the type of build surface matters; PEI sheets are popular because they provide excellent grip when hot and release the part easily when cool.