This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up

If all items have flex-grow set to 1, the remaining space in the container will be distributed equally to all children

If one of the children has a value of 2, that child would take up twice as much of the space as either one of the others (or it will try, at least)