A baryon is a subatomic particle, a hadron (a group of elementary particles that experience strong nuclear force) with a non-integer spin. Baryons accumulated aboard starships as a result of warp travel. They could be removed at facilities that performed baryon sweeps, a procedure lethal to organic material.
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Background information Edit
Baryons are heavy subatomic particles, including protons and neutrons. As almost all normal matter in the universe contain these particles, a process which eliminates baryons wouldn't leave much behind. We can assume that the baryon sweep refers to a specific type of heavy subatomic particle created by the subspace field of warp drive, rather than the entire family of them. A baryon sweep would thus indeed be dangerous to organic matter, for the reason stated above.
Michael Okuda and Rick Sternbach suggested that a "baryon field unifier" could be used to detect the Bajoran wormhole in a line cut from DS9: "Emissary". Michael Piller used "field density compensator" instead. The line was retained in the novelization. (The Making of Star Trek: Deep Space Nine)