tl;dr stated off as a way of saying "this is too long and therefore requires too much effort for me to read it." Then that gave rise to people accompanying long reads with a "tl;dr version," which is usually a one or two sentence summary. Now that the latter is common and understood people just write tl;dr and then follow it with the summary, with the understanding that those who are unwilling to read the full version will read that instead.
