Skillful means are the various methods used to teach/transmit Buddhism. The skill is in fitting the means to the student. It is said that there are 84,000 skillful means, since there are so many different kinds of people. This concept that there are many equally valid ways to teach/practice has not completely prevented the kind of deadly sectarian strife that afflicts other religious traditions, but it does manage to minimize it.
For more information see https://en.wikipedia.org/wiki/Upaya.