Mate or Mating is an action performed by pressing the appropriate button when available.
Mating is a part of the process to
Give Birth to a 
baby.
You have to form a couple of a female and a male hominid to mate and create a baby. The main way to form a couple is to bond through grooming but you can also give a 
precious stone to an unrelated clan member.
Once the couple is formed, you need to use the Call Mate command to bring your couple together to being the mating process.
It is important to note that your hominid can't mate with a parent or a sibling. You won't be able to form a couple with them and mating isn't possible.
Mechanics[ | ]
Pick one of the couple and move to a 
Sleep Spot. 
Lie Down on the sleep spot and use the Drop Child action to remove any babies from your back. Use the Call Mate action to bring your couple together.
The Mate action will then appear when the couple is at the sleep spot and you can press the appropriate button to begin the mating process. The screen will shift upwards and not let you see the actions being performed. The screen will then shift back down, and if a pregnancy happens, an onscreen pop up will display NEW PREGNANCY.
Mating doesn't always produce a baby. You have to check beforehand if the female is fertile. You can do this by using the Analyze action on the female and it will show a white circle and FERTILE. If the female is fertile, a pregnancy will happen from mating. If she isn't fertile, mating is just a pleasurable activity for the couple.
You aren't required to give birth right away. You can stay pregnant and not give birth. You can wait until you are in a safer location and/or done travelling so you don't have carry a newborn small baby.
You can then continue the process to give birth to the baby.
Notes[ | ]
Be aware that if you proceed to the next generation while you have at least one pregnant female, the pregnancy will be lost. It's recommended to give birth to all unborn babies before doing a generation change.