This, written in later life, only briefly mentions the thing Tesla invented that was most useful. Tesla figured out how to get AC motors started. See his U.S. Patents #459,772 and #464,466. The second one shows how to use a starting capacitor, which is how AC motors are started today.

AC motors were known before Tesla, but getting them up to speed with the power source was tough. It usually required a second motor with a commutator to get the AC motor started. Tesla figured out a simpler way, one that made AC motors simpler and cheaper than DC motors.

DC motors with commutators were easier to start and more controllable, but less reliable. Early motors had actual wire brushes, which arced, wore out, needed adjustment, and wore grooves in the commutator. Carbon brushes came later, and really good carbon/graphite brushes were not developed until well into the 20th century. It's one of those materials quality things that isn't thought about much any more, because it's a solved problem.