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Not only was he tremendously prolific, churning out 44 novels and 121 short stories in his lifetime - he died in 1982 aged 53 - but he was famously prone to hallucinations and paranoid delusions, even having something of a religious experience that revealed his son had a fatal birth defect. Doctors successfully saved his son's life, but only after Dick told them what to look for.
He toiled in semi-obscurity for a large part of his career, but Hollywood later discovered his work, adapting story after story into numerous big-name movies. It's why many consider him Hollywood's go-to source for sci-fi (Minority Report, Blade Runner, The Adjustment Bureau).
Dick's work addresses all kinds of topics, but it asks different versions of the same question over and over again: How do we know what's real and what's not?
As our world becomes increasingly virtual, the "real" can be easily threatened by the "not-real." Imaginary Bitcoins have people questioning their physical dollars. Daily communication happens in ones and zeroes via email rather than through physical letters by pen and paper. This same type of tension proliferated Dick's work and life.
He looked at a future where technology went unchecked by humanity, and he didn't like what he saw.
A Science Channel documentary called "Prophets of Science Fiction" (available on Netflix) takes a look at Dick's work through the technology he predicted and feared.