Gosh, that's a huge question. Plato contributed so many different things and in so many different ways that people have spent entire books summing them up. However, if I had to sum those contributions up in a list, I'd say the following:

His description of the role of the philosopher establishes precedent for the role of reflection.

His description of the death of Socrates established the ideal of the intellectual warrior.

His description of the Republic helped establish the tradition of the rational utopia.

His concepts of forms and the absolutes shaped Christian thought for thousands of years.

More generally, his philosophy established traditions of related to metaphysics, epistemology, and politics that emphasize absolutes, direct perception, and ethical integration of society.

He communicated his thought in fine works of literature (the dialogues), demonstrating that philosophical truth and literary beauty can be fused.

He founded the Academy, which formalized a tradition of having philosophy as a major element of formal education.