Kriyā yoga is the name Patañjali gives to three practices taken together: tapas, svādhyāya, and īśvara-praṇidhāna. He names them in 2.01, the opening line of the chapter on practice, before he introduces the eight limbs. The same three return later in that chapter as the last three of the five niyamas.
Kriyā on its own means action. Adding yoga makes the three a practice you do rather than a subject you study.
Kriyā yoga is not karma yoga, though the two are connected. Karma yoga, which comes from the Bhagavad Gita, describes a whole path: doing your work and releasing the fruits of it. Kriyā yoga is narrower: three practices within Patañjali’s system. Both are concerned with action and with letting go of results, but they come from different texts and do different jobs.
The significance of kriyā yoga is disputed. Some commentators read it as preparation, a starting point for practitioners not yet ready for the eight limbs. Georg Feuerstein, in the Preface to his book The Philosophy of Classical Yoga, writes that Patanjali's "own view seems to be that kriyā-yoga ... can be equated with Classical Yoga per se." The sutra itself does not settle the question.