It does not really get explained any more simply than this:

One day when our master Jamyang Khyentse was watching a lama dance in front of the Palace Temple in Gangtok…he was chuckling at the antics of the … clown who provides light entertainment between dances. [A Student] Ana Pant kept pestering him, asking him again and again how to meditate, so this time when my master replied, it was in such a way as to let him know that he was telling him once and for all: “Look, it’s like this: When the past thought has ceased, and the future thought has not yet risen, isn’t there a gap?” “Yes” said Ana Pant. “Well,  prolong it. That is meditation”.

Sogyal Rinpoche, Glimpse after Glimpse.