A steel man is an argument that has been improved as much as possible – "armored" or "steel-plated" – so as not to have any unnecessary vulnerabilities, while still attempting to faithfully represent the same essential idea.
It is the opposite of a straw man.
RationalWiki explains it thusly:
Steelmanning keeps us rational, reminding us that we’re arguing against ideas, not people, and that our goal is to take down these bad ideas, not to revel in the defeat of incorrect people.—Chana Messinger, The Merely Real
Steelmanning is the opposite of the strawman fallacy. A steelman argument occurs when one intentionally seeks the best form of the opponent's argument, or deriving the strongest possible position from their current arguments. Using this approach encourages pure discussion rather than leading to just another exchange of rehearsed or quietly Googled counterpoints and cheap cop-outs, demonstrates your knowledge of the discussion at hand, and basically makes you a better debater and a better person in general.
When employed in conjunction with a skilled devil's advocate, steelmanning can be an excellent technique for assessing one's own position or ideology for weaknesses. Once these weaknesses are known, the position or ideology can be re-engineered to be more in conformance with reality, and, consequently, more resistant to rational refutation.
- The Virtue of Steelmanning, Camels With Hammers