Well, they pretty much answered your question. I'll go word per word to expand a little.
estoy = comes from verb 'ser'. it is the first person singular form along with 'yo soy'.
in spanish the verb 'to be' takes 2 forms. either a form which defines a place you might be on, or a 'state' you might be having, or a form that can take adjectives, and means some quality you might have. remember the pronoun is usually omited.
'yo estoy' = i am, referring to a place you might be at, or a 'state' you might be in (mood, health, etc)
estoy en buenos aires = [ i ] am in buenos aires
estoy feliz = [ i ] am happy
'yo soy' = i am, referring to a quality (be it either negative or positive) you might have. it's also used when you're telling someone your name ('who you are')
yo soy rapido = i am fast
yo soy kylecito = i am kylecito
yo soy siempre lento = i am always slow
Hope this helps! if you got any questions feel free to ask