Like in the other terms of a relationship status, (bases 1-5), 'sixth base' is a way of saying that a relationship is at its highest and most intense point. The couple could be having extremely romantic and pleasurable intercourse, in which case one of them will state that they've experienced 'sixth base', like one says which base they've been to in other cases.
'How far have you been?'
'I've been to third base'
'How's it going with Lisa?'
'Awesome, last night we went to sixth base!'