A herb with a broad therapeutic range is safe. That means the difference between the dose that has a medicinal action and a dose that can give side effects is very large. Most herbs have an extremely broad therapeutic range. It is very difficult or even impossible to take a toxic dose of most herbs. Chamomile, for example, can be taken in small or large quantities with no adverse effects.
On the other hand:
There are herbs that must be used with caution because their therapeutic range is narrow. For example, Lily of the Valley affects the heart in a similar way to Foxglove which is poisonous. Only a little Lily of the Valley is needed to have a beneficial effect, too much would be toxic so it is important to make sure an accurate dose is taken. A more common example of this is coffee – which can make us feel jittery and anxious if we drink too much.