Film sensitivity to light — higher ASA means you need less illumination. 100 for bright sun, 400 for overcast, 800+ for night work.
The ASA speed rating determines how sensitive your film stock is to light—and therefore, how much exposure you need to get a usable shot. The higher the number, the less light is required. In practice, this means: on a sunny day, you'd use a 100 or 200 speed film; under overcast skies, you'd bump up to 400; and at night or in dark interiors, you'll need 800, 1600, or even higher—provided your camera supports it.
Your camera's light meters—whether TTL or handheld—calibrate to the ASA speed rating you input. If you set it incorrectly, your meter is blind: too low an ASA means overexposure (film becomes milky and flat), too high leads to underexposure (dark, grainy images with little shadow detail). This is critical in film because you can't correct it in post-production like with digital—the film records what it records. The classic Kodak standards were 64, 100, 200, 400, and 800; modern emulsions go up to 3200 or higher.
On set, you'll quickly notice: higher ASA speeds mean more grain—this is physical: the silver halide crystals in the film become larger to become more sensitive to light. A 1600 speed film has visibly more texture than a 100 speed film, which may be desired or undesired depending on the aesthetic. Some DoPs intentionally use high-speed films for the look, while others want the clarity of slow stock and build up massive amounts of light instead—this costs time and money, but pays off in image quality. In digital production, the equivalent is the ISO setting, but it works differently: with digital, you can change ISO between takes, but not with film.
A practical tip: if you're working with color film and are unsure, go one stop higher than your light meter suggests—film often tolerates overexposure better than underexposure, which otherwise loses detail. With black and white, you have more leeway because the tonality is more robust. And remember: the ASA speed rating is not variable like with video—you choose your film stock, its characteristics, and that becomes your partner for the entire shoot.