My understanding as my therapist has explained it to me, is that she can 'contain' my emotions, my acting out, my mood swings etc. without reacting back at me. She is holding my emotions and still remaining calm and strong and in control. As with my children, if they were to get angry with me and declare that they no longer liked me, rather than me saying I no longer liked them, I would contain their emotions and react lovingly back towards them. (they are young children, not teenagers or young adults). I believe this is how the therapist needs to be towards their clients, the eternal 'container', where you can say whatever it is you feel (within reason) and there is no retaliation.

I hope this helps.