Flexbox and CSS Unit Values Practice (QR Code Challenge)

I need help understanding why using height: 100% caused my project to be wonky with the body spilling out of the card area, but using height: 100vh fixed it in the code below.

    background: hsl(212, 45%, 89%);
    font-size: 15px;
    font-family: 'Outfit', sans-serif;
    text-align: center;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    height: 100vh;
}
.card{
    background: hsl(0, 0%, 100%);
    border-radius: 1rem;
    max-width: 17rem;
    padding: 1rem;
    margin-bottom: 1rem;
}

Any tips for learning more about viewports would also be helpful.