Yes—brisk walking counts as cardio (aerobic activity) when it is moderate intensity: your heart rate and breathing rise, and you can talk but not sing comfortably. The Physical Activity Guidelines for Americans define aerobic (or cardio) activity as rhythmic, sustained movement of large muscle groups; brisk walking is listed alongside running, bicycling, and swimming as a classic example.
Adults should aim for at least 150 minutes a week of moderate-intensity aerobic activity (or 75 minutes of vigorous activity, or an equivalent mix). Brisk walking at about 2.5–4 mph is treated as moderate intensity (roughly 3–6 METs), while a slow leisurely stroll is generally light activity and does not meet that moderate-intensity bar by itself.
You can accumulate the minutes across the week—for example, five 30-minute brisk walks. Intensity is personal: the same pace can feel moderate for one person and vigorous for another, depending on fitness.
Image: U.S. Army photo by Sgt. Agustín Montañez; public domain, via Wikimedia Commons.