Ki's are simply their powerlevel.They fight and they grown their Ki in order to be faster,get stronger.Saiyans everytime they fight their Ki grows and they become even more powerful.Also Saiyans once they reach a certain level,they turn SSJ.Most just have a level to reach to and that's it.An example of their Ki is when the DBZ gang is fighting Frieza and Frieza transformed for the first time and he's powerlevel grew up to 1,000,000!!,and all those times their powering up.So now you know what a Ki is.
