Slowly explains the verb's how part. You can also explain the when part or where part. So, when you say "I'm slowly wiping the floor tomorrow", you're explaining the how part and when part of the verb.
It's important to note that "rano", when used as an adverb, doesn't mean "morning" in English, but rather "in the morning", except in the case of "jutro rano" which literally translates to "tomorrow morning". So, you can say "Idę rano" in Polish but not "I am going morning" in English - "rano" can act as both noun and adverb, while "morning" can not. (experts, correct me if I'm wrong somewhere)
The first thing : Polish is not alone in this. Compare French: aujourd'hui, matin, hier and so on.
Second: even in English you have this words describing time: recently, lately, long ago etc.- all of them adverbs.
In Polish there are some adverbs used as descriptors of time: rano, rankiem, wieczorem, nocą, dziś, dzisiaj, wczoraj, przedwczoraj, jutro, pojutrze, przedpołudniem, popołudniu