Captains are expected to wait a certain amount of time before they are allowed to cancel the ride, and carry on accepting other bookings. You can always call your Captain to let him know you will be late, and ask him to wait a little longer.
If the Captain cancels after waiting the full release time, you may be subject to cancellation charges.
For details on the release times and cancellation charges according to your city, please check the Help Centre on the Careem app. Go to the ‘Car’ tile on the app, then Help > What are Careem’s rates to check out the details on the Captain release times according to a city.