Most apps work with algorithms that make predictions based on your past cycles. In other words, they estimate the date of your next ovulation (and therefore your next period) based on the ovulation day in your past cycles.
But remember: ovulation CANNOT BE PREDICTED. By charting your fertility signs, you can confirm it happened, but it’s impossible to predict when you will ovulate in the next cycle. This is especially important if you’re trying to avoid pregnancy (or get pregnant, for that matter).
Keep in mind that ovulation is quite sensitive. Stress and other factors can delay it (and therefore delay your period), and these are things your app can’t anticipate.