What does $\{$ a set $\}^{+}$ mean in the context of languages?
This is the Kleene plus. It stands for
$$ L^+ = \bigcup_{i \geq 1} L^i. $$
Here $L^i$ is the set of concatenations of $i$ words from $L$.
In words, $L^+$ consists of all concatenations of one or more words from $L$. A related operator is the Kleene star
$$ L^* = \bigcup_{i \geq 0} L^i, $$
which also allows the empty string ($L^0$).

For example, if $L = \{a\}$ then $L^+ = \{a,aa,aaa,aaaa,\ldots\}$ while $L^* = \{\epsilon,a,aa,aaa,aaaa,\ldots\}$. If $L = \{a,b\}$ then $L^+ = \{a,b,aa,ab,ba,bb,aaa,\ldots\}$. If $L = \{aa,b\}$ then $L^+ = \{aa,b,aaaa,aab,baa,bb,aaaaaa,\ldots\}$.