Any message can be represented as a number. For example, the word "meet" in "Let's meet at the corner of Constitution and Lake" would be represented in most computers by 109 101 101 116 in decimal, using the encoding known as ASCII.
Suppose you want to send a secret directive--perhaps a rendezvous time and location--via five couriers, but you fear that one or two will be caught. You therefore want to spread the message among the couriers such that any three of them can together reconstruct it but two or fewer cannot.
This article was originally published with the title All or Nothing.