On it's own, its "Space, interval; length, distance, extent". But most of the time it's not on it's own - it's used with the copula (is ea, ní hea) and the FGB says that it's a "neuter pronoun", and provides lots of examples. Is ea is as close to "yes" as it get's in Irish (though "it is" might be a better rendition).
'is ea' is used to stress or emphasize, in this case 'teachers. You can say, 'Is múinteoir mé' but you aren't stressing/emphasizing any word in particular. When you say, 'Múinteoir is ea mé', you are emphasizing your particular profession (out of pride) or possibly correcting someone who thought you did something else.