The No-Prize was a fake award that readers would get from Marvel Comics. You could get it for having done a "meritorious service to the cause of Marveldom" (such as finding or explaining mistakes in continuity). The term "No-Prize" comes from Fantastic Four issue 26, where Stan Lee wrote (in a letter printed in the issue) "there will be no prizes, and therefore, no losers."
It was the practice of other companies to give a prize to people that found mistakes or wrote a letter. Marvel fans asked Marvel to do the same. As a joke, Stan Lee (starting in 1964) decided to give people, who had perform some form of "meritorious service to the cause of Marveldom", a letter that told them what they won a No-Prize i.e. nothing at all. While at first he gave it to people that wrote creative responses to questions he posed in the letter column, Stan Lee decided to take a new approach. As a parody of the practice that inspired it, he gave these out to people that found such mistakes in Marvel publications. Despite intending the No-Prize to remind Marvel readers to just relax and enjoy the stories, letters sent to Marvel tripled. In response to the people asking why they didn't get an award, Marvel Comics, starting in 1967, sent envelopes with the words "Congratulations, this envelope contains a genuine Marvel Comics No-Prize which you have just won!" printed on it. In 1982, Marvel produced The Official Marvel No-Prize Book (which points out and mocks Marvel's past mistakes). In 1986, editor Mark Gruenwald discontinued the practice since he felt it had negatively affect the letters sent to letter columns. In 1989, Ronald Perelman (would drive Marvel into bankruptcy) called the No-Prizes "a silly, expensive extravagance to mail out." Despite this, Tom DeFalco brought the practice back in 1991 (when he was editor-in-chief) and made a new criteria: that in order to get a No-Prize, you had to perform a "meritorious service to Marvel above and beyond the call of duty".