AMO stands for archery manufacturer's organization. its the standards used to measure bows so you can go from one shop to the next and say "ive got an amo 63 inch recurve and need a string for it" and the shop owner will give ya the string you need.
most likely you will not want a fastflite string unless you know for positive your bow was made to take the extra abuse. its kinda like a +P rating for bowstrings. generally fastflite strings allow more energy to be transfered to the arrow and only bows made specifically strong enough should use them.
you will prolly need a b50 string for a recurve of amo lemgth 63.
i highly recomend getting "shooting the stickbow" book from anthony camera. http://www.shootingthestickbow.com/
its got everything and then some for any beginner to shooting recurves or longbows