The CSS grow-in effect, also known as 'in' or 'transition-in', allows elements to change their appearance over time when hovered. This effect can be used to create a smoother transition between states of an element.

css
transition: opacity 0.3s ease-in-out;
:hover {
  opacity: 1;
}

To apply the grow-in effect, you need to define a transition property on the element and then use the :hover pseudo-class to change its properties over time.

The transition property specifies a timing function that controls the speed of the transition. In this example, the transition lasts for 0.3 seconds and uses an ease-in-out timing function.

css
.object {
  transition: transform 1s linear;
}
(object:hover) {
  transform: rotate(360deg);
}

By using the grow-in effect, you can create a more engaging and interactive user experience for your website or application.