Frankl talks about what gives life meaning. Normally I would quibble about the use of the term 'meaning' but I think it's clear what he means - the factor that makes it worthwhile to continue to exist, and to make a wholehearted commitment to life, when things are bad - and for him they were obviously as bad as they could be.
Obviously I've just got to simplify to a stupid extent, but it seems to me he's saying that people can find this meaning in the simple pleasures of life, and in relationships with others, and in creativity and self-expression. But when all this is gone - and for some people it is - then they can find meaning in suffering. In brute suffering survival.
I get very anxious when people say that suffering can give meaning, because often it is a prelude for doling out that suffering in massive life-enhancing lumps, always to other people, for their own good. Frankl is nothing like that. He says that if suffering can be relieved it should be, either in oneself or in others. However, when suffering is inescapable, for instance for those with severe disability or chronic pain, it can become in itself a source of meaning.
In arguing this case he sometimes uses religious language that I don't find conducive. For instance that your suffering may burn away your sin to make you fit for heaven, I don't think that is healthy or helpful.
On the other hand when he speaks about our task of carrying consciousness through the universe, as bearing witness to existence, this seems meaningful to me. I think it is healthier than the modern view which identifies meaning with success and achievement, and hence consigns millions of people to meaninglessness.