It is the first person singular form of the verb "estar", the Spanish verb meaning "to be". So it means "I am".
'I am' in relation to temporary states, e.g. location, emotion
Estoy en el tren (I'm on the train)
Estoy muy feliz (I'm very happy) (at the moment)
By contrast 'soy' from the verb 'ser', also meaning 'to be', relates to more permanent or innate states, e.g.
Soy ingles - I am English
Soy feliz - I am happy (by nature, a happy person)
''Estoy bien'' is the Spanish for ''I am fine''.
estoy
estoy cansado means "i am tired" in spanish
"Estoy mintiendo" means "I am lying" in Spanish.
The Spanish phrase "yo estoy aca" translates to "I am here" in English.
"Estoy pensando" in Spanish translates to "I am thinking" in English.
Estoy means I am, particularly relating to location and other temporary states.
Estoy bueno.
I'm great!
"Where the hell am I?"
Estoy bien.
In Spanish, "y aquí estoy" translates to "and here I am" in English.