Add a resting heart rate for more accurate zones (Karvonen method), or leave it blank for simple %-of-max zones.
Max heart rate is estimated with the Tanaka formula (208 − 0.7 × age) unless you enter a known value. These are general guidelines — individual max HR varies.
The most accurate way is a supervised field or lab test, since individual max HR varies a lot and can differ noticeably from age-based estimates. Without a test, an age-based formula like Tanaka's (208 − 0.7 × age) is a reasonable starting estimate.
The %Max HR method sets zones as a straight percentage of your max heart rate. The Karvonen (heart rate reserve) method also factors in your resting heart rate, which better reflects individual fitness and tends to be more accurate, especially at the easier end of the zones.
Measure it first thing in the morning before getting out of bed, using a heart rate monitor or by counting your pulse for 60 seconds. Averaging a few mornings gives a more reliable number than a single reading.
For most runners, the majority of weekly training time is spent in the easier zones (Zone 1-2), with harder zones used more sparingly for specific workouts. Exact distribution depends on your goals and training plan.