According to the RSS specification, “ttl stands for time to live. It's a number of minutes that indicates how long a channel can be cached before refreshing from the source. This makes it possible for RSS sources to be managed by a file-sharing network such as Gnutella.”
No one is quite sure what this means, and no one publishes feeds via file-sharing networks.
Some clients have interpreted this element to be some sort of inline caching mechanism, albeit one that completely ignores the underlying HTTP protocol, its robust caching mechanisms, and the huge amount of HTTP-savvy network infrastructure that understands them. Given the vague documentation, it is impossible to say that this interpretation is any more ridiculous than the element itself.