Firstly means at the beginning or first in time or order. It's often used as an adverb.
When speaking or writing about several points that need to be explained, people usually start with firstly because it helps them introduce their main idea before going into detail. For example, if you're making a list of reasons why you love your hometown, you might say "Firstly, the food here is amazing" and then explain what makes the local cuisine so great. You can also use firstly to emphasize something that's particularly important or interesting.
In the first place; before anything else; first.
