These tests screen for something called the LH surge. When this is detected, ovulation should occur within approximately the next 24 hours. This can help you time when to have sex so you can optimize your chances of getting pregnant.
Just before ovulation, your body increases the amount of luteinizing hormone that is released, also known as LH. When it peaks in amount, this is called the LH surge, and this surge is what triggers ovulation. Since the LH surge usually happens about 24 hours before ovulation, detecting it can help couples know when is the best time to have sex—which in general is the day of the LH surge and the day after.
Ovulation predictor kits are over-the-counter tests that screen for the LH surge. They use urine to detect the LH, much like how a pregnancy test screens for the hormone human chorionic gonadotropin. Some women find these tests helpful to know exactly when they are ovulating, while others may feel they are unnecessary in the trying-to-conceive process.