What does "estoy" mean?
'Estoy' is the first person singular form of the verb 'estar,' and it means 'I am,' in the sense of being in a place or being in a temporary condition.
'Estoy en México,' can be translated 'I am in Mexico.' That's in the sense of being somewhere. 'Estoy cansado,' could be translated 'I am tired,' or 'I feel tired.' That's the temporary condition sense of the word.
'Soy' is the first person singular form of the verb 'ser,' and it also means 'I am,' but in a different sense. The difference between 'estar and 'ser' is a difficult thing to learn at first, and the Spanish course I am taking describes it like this:
'Estar' is used to show location or position: where the subject is. 'Estar' is also used to show condition or state: how the subject feels. These I have given examples for.
'Ser' is used to indicate origin, possession or material. - what country the subject is from, ie. 'Juan es de Guatemala.' 'Juan is from Guatemala.' - to whom the subject belongs, ie. 'El libro es de Isabel.' 'The book is Isabel's.' - of what the subject is made, ie. 'La lámpara es de cristal.' 'The lamp is made of glass.'
'Ser' is used to indicate basic characteristics or identisfication. - describes the subject, ie. 'Ella es joven.' 'She is young.' - recognises or specifies the subject, ie. '(Nosotros) Somos estudiantes.' 'We are students.' - tells the use or function of the subject, ie. 'Los lápizes son para escribir.' 'The pencils are for writing with.'
'Ser' is used to indicate profession or nationality. - what job the subject does, ie. 'Él es electricista.' 'He is an electrician.' - of what nationality the subject is, ie, 'María es mexicano.' 'Maria is Mexican.'
'Ser' is used to indicate time of day, date or location of an event. - what time it is, ie. 'Son las tres de la tarde.' 'It's three in the afternoon.' - what the date is, ie. 'Hoy es diecisiete de febrero.' 'Today is the seventeenth of Febraury.' - where an event is located, ie. 'La fiesta es en el parque.' 'The party is in the park.'
That's pretty comprehensive, I think, and I'm sorry if I oversimplified it. I needed a refresher course on that too!