Wildcard | Define Wildcard at Dictionary.com
A "wildcard" team, for example in American football, qualifies for the Super
Bowl tournament despite not being the #1 team in their division. Each of the 4 division winners make the "playoffs." The two best teams that did not win their divisions also qualify as "wildcard" teams.
Your message here seems to be about "American Idol." In this case, it means that those who have "lost" on the show have been given another chance to win. Wildcards are those who are strong competitors who may have failed to win early in a competition, but remain a contender for winning the prize.