These numbers represent the number of cycles in a puretone burst sound signal commonly used in Auditory Brainstem Response (ABR) testing for hearing threshold estimation. The ABR relies upon an abrupt onset sound signal to produce sufficient neural synchrony for an evoked response to be measurable using surface electrodes. However, an abrupt onset signal like a click contains sound energy across the audiological range and so cannot easily differentiate different patterns of hearing loss (e.g. high frequency sloping, flat, low frequency sloping). Hence the technical requirement for abrupt onset signals must be weighed against an audiological need to use frequency specific stimuli, so as to resolve different patterns of hearing loss. The “2-1-2” (or “2-2-2”) represents the number of cycles of a pure tone in the onset-steady state-offset regions of the stimulus envelope. For example, in the below figure are two pure tones of five cycles. The top panel shows a pure tone that is steady state (i.e. the amplitude of each cycle is the same) whereas the pure tone in the lower panel is rising (2 cycles) to steady state of 1 cycle before falling again (2 cycles). The total duration of a signal depends on the stimulus frequency.
You may have surmised from the comments above that the short duration of the toneburst means that it will not have the frequency specificity of a relatively long duration pure tone used in pure tone audiometry.
The relatively abrupt onset and offset produces “spectral splatter” or energy that occurs at frequencies other than the nominal audiological test frequency. The precise pattern of the spread of energy to other frequencies depends on the duration of the stimulus and the shape of the stimulus envelope. If you would like to learn more about the frequency content of different tonebursts then there are many excellent sources, for example chapter 5 in this popular text: Picton (2011) Human auditory evoked potentials Plural (CA).
Related course: Beyond the basics: Threshold ABR