Something that's boundless has no limits or restrictions. Your dog's boundless energy might leave you exhausted, since it never seems to end.
The adjective boundless is often used to mean "huge" or "immense." Your local Christmas tree farm, for example, might seem boundless as you walk through the rows of evergreens that stretch as far as you can see. If your little brother has boundless enthusiasm for superheros, he's probably capable of talking about Batman for hours. The noun bound originally meant "limit," from the Old French word bonde, "limit or boundary," so boundless means "without limit."