What does this language notation specify?
Language concatenation is defined as follows:
$$
L_1 L_2 = \{ w_1 w_2 : w_1 \in L_1, w_2 \in L_2 \}.
$$
In other words, a word $w$ belongs to $L_1 L_2$ if it can be written as the concatenation of two words, $w_1 \in L_1$ and $w_2 \in L_2$.
This definition should suffice for solving the exercise.