Isochronic comes from two words "iso" meaning equal and "chrono" which relates to time. From that we can gather that isochronic tones are sounds that occur at intervals equal to the duration of the tone. For example if a guitar string is plucked and is muted after 5 seconds then plucked again after 5 seconds and is done repeatedly - you'll hear an isochronic tone. You hear the tone for 5 seconds then you hear it again after 5 seconds. Here's a visual representation:
Think of the wavy pattern as a tone and the horizontal line as the interval. The duration of the tone and the interval are equal. That's an isochronic tone which has been used for brainwave entrainment.
Our brains generate different brainwave frequencies at different levels of consciousness. Brainwave frequencies during sleep are different from brainwave frequencies occuring while we are fully awake. Using isochronic tones, our brain can be entrained to a specific brainwave frequency in a short period of time harnessing the benefits that the specific brainwave frequency brings.