In most cases, you are most fertile and should have intercourse when the Fertility Analyzer gives you a green light (it uses all the signs you enter to determine your fertility status).
Red crosshairs will be drawn on your chart once the Fertility Friend app detects that you have ovulated. If you experience a cycle where your signs are ambiguous (your body doesn't always follow any set of expected rules, and charting conditions are not always ideal) the app may display a dotted line to suggest that the interpretation is less certain. This is meant to alert you to keep looking for signs of fertility so you do not miss a chance.
Here's a brief overview of the hormonal process and its relationship to your fertility signs: (Your own hormonal profile may vary somewhat).
Your cervical fluid gets increasingly wet and slippery as you get closer to ovulation. When it is most slippery and wet, that is when you are most fertile. This increase in cervical fluid production is caused by the hormone estrogen, which is released by the ovarian follicle as it is maturing and getting ready to release an egg at ovulation. You should have intercourse when you have this kind of fluid because it indicates that ovulation is approaching and it nourishes and transports sperm while they are waiting for the egg to be released. Ideally, you want to have sperm waiting in your reproductive system when the egg is released.
Cervical fluid can dry up a day or two before, or the day of ovulation or even the day after ovulation, so you can't use it as a sign to pinpoint ovulation, though it is a great indicator of fertility. Likewise, OPKs (ovulation predictor kits) if you use them, can help you determine the best time to have intercourse since they are usually positive (indicating an increase in luteinizing hormone- the last hormone to peak before ovulation and the hormone that triggers the rupture of the ovarian sac, causing the egg to be released) 12 to 48 hours before you ovulate.
You will only know that you have ovulated, though, when you see that your temperature has risen. Once you have a clear and sustained thermal shift you can be reasonably sure that you ovulated on the day before the shift. You can know this because after ovulation, the corpus luteum (what's left of the ovarian follicle that released the egg at ovulation time) begins to produce progesterone, a heat-inducing hormone that causes your body temperature to rise slightly.