When people talk about a meal ticket, they usually mean a person, thing, or skill that provides income. For example, you might describe your ability to knit cute arm warmers as a possible meal ticket.
If you marry a wealthy person, friends might gossip that you're using her as a meal ticket, but they're just as likely to compliment your jazz piano skill as a meal ticket. The phrase originally meant, literally, a paper coupon that could be redeemed for food. Beginning in the late nineteenth century, young workers often dined using meal tickets that provided a week's work of discounted lunches at restaurants. The figurative meaning came soon after.