The Buddha says in a gatha:

I am the Dharma King,
With respect to the Dharma acting completely at will.

The Buddha, the Dharma King, grasps the true nature, the ultimate dimension, of all things (dharmas) and therefore has the ability to use various skillful means to teach beings in the phenomenal realm – this world of form and appearances called “samsara.” The various teachings are Dharma doors, and a Buddha is someone who can enter any of these Dharma gateways at will and use them in a very free and skillful way, just as a great poet knows how to use words with great artistry and skill. So the teachings may appear in different forms, but ultimately they all lead to the One Vehicle, the Buddha vehicle, in which all beings realize their innate Buddha nature. This is absolute freedom in the field of time and space, nirvana right in the realm of samsara, and this is the great insight of the Buddha that was renewed in the Mahayana.