Real Dictionary

too adding something or responding
adverb: You use too after adding a piece of information or a comment to a statement, in order to emphasize that it is surprising or important.
adverb: You use too after mentioning another person, thing, or aspect that a previous statement applies to or includes.

too in American English
in addition; as well; besides; also
adverb: in addition; also; furthermore; moreover

too in British English
adverb: as well; in addition; also

too indicating excess
You use too in order to indicate that there is a greater amount or degree of something than is desirable, necessary, or acceptable.
You use too with a negative to make what you are saying sound less forceful or more polite or cautious.