Something that's wholehearted is completely sincere and genuine, like your wholehearted support of a friend who's running for school president or her wholehearted commitment to running a positive campaign.
The adjective wholehearted comes from the idea that you're putting your entire heart into something, with all the feeling and sincerity that implies. Your wholehearted apology for accidentally tripping someone is earnest and heartfelt, and your wholehearted attempt at winning the 50-yard dash is focused and determined. Anything that's done with that kind of commitment is wholehearted.