What exactly does it mean when martial law is inacted in a city in a state of emergency?

I heard that in Pleasant Grove, Alabama that martial law has been encted and they are covered up with military personnel. What does the military do there?
Update: I must have misheard or gotten misinformation about the enacting of martial law. But it made since when I saw all the gaurds in their uniforms and everything. Thankyou so much for all of your input.
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