The flex-basis CSS property is used in flexbox layouts to define the initial size of a flex item along the main axis before any remaining space is distributed. It specifies the ideal size of the item, taking into account the available space and other factors like flex-grow and flex-shrink. The flex-basis property can be set to a specific length value (e.g., 200px), a percentage (e.g., 50%), or the auto keyword.
For example, flex-basis: 300px; would set the initial width of a flex item to 300 pixels.