Je t'aime is understood as 'I love you' because French has two forms for 'you'. One is familiar; it is used for family, close friends, lovers and God; that is 'tu'. 'Vous' is used for everyone else. Like 'you' in English it serves for singular and plural. Tu is rather like 'thou' once was and is singular. So a lover says 'je t'aime'. Aimer can be either 'to love' or 'to like'. Someone who said 'je vous aime' would be understood to mean 'I like you' ( otherwise they'd use tu in this context ).