This is the 86th most frequent Hungarian word.
közül
"From among" or "out of."
In this sentence, 'közül' indicates a choice within a group, 'his friends'.
A barátai közül Péter volt a legkedvesebb.
Among his friends, Peter was the kindest.
'Közül' is used here to signify selection from a set, 'the tasks'.
A feladatok közül a legnehezebbet oldotta meg először.
Out of the tasks, he solved the hardest one first.
Here, 'közül' is used to denote selection from a group, 'the books'.