RANDOMIZE fixes what the random number sequence that RND uses will be. If you RANDOMIZE a number more than zero, it sets the same (for that number) series of 'random' numbers every time. If you RANDOMIZE 0 (or just RANDOMIZE) it sets a different series every time based on the spectrum's clock. If you try the following with RANDOMIZE number versus RANDOMIZE you'll see the difference. The former will give the same sequence every time you RUN, the latter will be different every time.
10 RANDOMIZE 50
20 FOR A=1 TO 10: PRINT RND: NEXT A
