A transition is a smooth change from one visual state to another. It is like a lamp slowly brightening instead of snapping from dark to bright in one instant.
On a page, a panel might slide open when someone presses a button, or a color might fade to show a link is being hovered. The change can help people follow what happened.
Keep movement short and purposeful. If the transition is slow or decorative without helping understanding, it can make the site feel sluggish. Some people also prefer less motion, so respect that setting.