When you are fertile it means that is when it is easiest for you to conceive
Are you talking about on an app? This is how a typical woman's cycle works: Day one of your period is Cycle day ONE. You count from there. Most women ovulate around day 14. Ovulation lasts 24 to 48 hours max and when it is over, it is a less fertile time as no egg is there to fertilize (although there is NO safe time for unprotected sex as you could have a rare second egg or ovulate later than you thought you would . . .). The days leading up to day 14 of your cycle are considered fertile days because sperm can live inside of you for a week. So a whole week before day 14, that sperm can impregnate you. Starting at day 9, pregnancy can occur. You continue counting for that cycle until the first at of your next period. The number of days in between day one and the day before your next period is your complete cycle.
AT your age though, you may be very irregular which is completely normal and quite common. This means you can have periods at irregular times long cycles, short cycles, etc. It's hard to use an app that averages for any type of fertility prediction when you are irregular. But you can at least track when your periods are and have record of it. good luck