Israel ben Eliezer is known through hundreds of hagiographic tales, most collected in Shivhei ha-Besht fifty years after his death, and for a long time that was effectively all there was.
Moshe Rosman changed the position in the 1990s by going to the Polish archives, tax records, communal registers, the documents of the town of Miedzyboz, and finding the man in them. What emerges is a real and modestly documented figure: a baal shem, a master of the Name, which was a recognised occupation involving healing, amulet-writing and exorcism; supported by the community, which housed him rent-free, indicating recognised standing rather than the outsider status the later legend prefers. He was not the marginal wonder-worker of the stories, nor was he running a mass movement in his lifetime.
The teaching, transmitted through disciples since he too wrote almost nothing, turned on divine immanence: God fills all worlds and there is no place void of Him, which makes ordinary life, eating, working, ordinary speech, a possible site of worship rather than an interruption of it. Devekut, cleaving to God, ceased to be the preserve of the ascetic elite and became available in fervent prayer and in joy. Against a rabbinic culture that measured worth by Talmudic learning, this was a claim that the unlettered could reach God directly, and it is why the movement spread among ordinary Jews and why the established rabbinate opposed it so hard.
Hasidism as an organised movement is really the achievement of the generation after him, particularly Dov Ber of Mezeritch, and the retrospective construction of the Baal Shem Tov as its founder is part of that generation's work.