Randori means free-play. In other words someone isn't just going to fall over for you (embu or kata) you're going to have to make them fall over!
Randori is great fun, but also extremely difficult to do well because the other won't keep still! This is where all that kihon no tsukuri - timing, distance and balance breaking - comes into its own!
In a normal class situation it isn't necessary to take randori beyond hikitategeiko where your partner assists you in making the right choices, but if you want to take it further the training has to be specific and selective and a reasonable level of fitness is required.
At this club we have a session on Monday of just randori, 1h30 of trying to stay on your feet! On Thursday we have a session of taught randori, where Scott tries to put right all the crimes we were committing at the Monday session!