Something seamless could literally be a piece of clothing without seams, but it's usually something else that's smooth or unbroken, like a seamless transition.
Seamless things are connected so well that you can't see what's holding them together. They're flowing, consistent, and well-put-together. If an employee leaves, and the replacement does a great job immediately, that's a seamless transition. In basketball, if a team goes from offense to defense without missing a beat, that's seamless play. And that underwear that seems to be made out of one piece of material? Seamless. And super comfortable.