Limits to growth means that there are some things that we can't keep doing forever. For example, if we use all our water to have a bath, eventually it will run out and we won't be able to have any more baths, so this is a limit to how much we can use. Similarly, if we chop down all the trees, eventually we will have none left, so this sets a limit to how many trees we can use. In general, there are always limits to how much of something we can use or how much something can grow.