What are access structures in Attribute-Based Encryption? I'm reading this article, but I could'nt figure out Definition 1 (Access Structure) on page 3, section 3.1.
It says:

Let $\{P_1, P_2,...,P_n\}$ be a set of parties. A collection $\mathbb{A} \subseteq 2^{\{P_1, P_2,...,P_n\}}$ is monotone if $\forall B,C:$ if $B \in \mathbb{A}$ and $B \subseteq C$ then $C \in \mathbb{A}$. An access structure (respectively, monotone access structure) is a collection (respectively, monotone collection) $\mathbb{A}$ of non-empty subsets of $\{P_1, P_2,...,P_n\}$, i.e., $\mathbb{A} \subseteq 2^{\{P_1, P_2,...,P_n\}}-\{\emptyset\}$. The sets in $\mathbb{A}$ are called the authorized sets, and the sets not in $\mathbb{A}$ are called the unauthorized sets.

First, what do they mean by access structure? And what is the power of a set of parties ($\mathbb{A} \subseteq 2^{\{P_1, P_2,...,P_n\}}$)? Is it meant as a power set? Could you give me an explanation about that, and/or give some examples?
An access structure is the set of sets that should have access. For example, if the set of parties is $\{P_1, P_2, P_3, P_4\}$ and the access structure, $\mathbb{A}$, is
$$\{\{P_1, P_2\}, \{P_3, P_4\}, \{P_1, P_2, P_3\}, \{P_1,P_2,P_4\},\{P_1, P_3, P_4\}, \{P_2, P_3, P_4\}, \{P_1,P_2,P_3,P_4\}\}$$
then $P_1$ and $P_2$ together have access, $P_3$ and $P_4$ together have access, but $P_2$ and $P_3$ together don't. Note that this access structure is monotonic. Since $\{P_1, P_2\}$ has access, $\{P_1, P_2, P_3\}$, $\{P_1, P_2, P_4\}$, and $\{P_1, P_2, P_3, P_4\}$ all have access.
Yes, it is meant to be the power set. $\mathbb{A}$ is a set of sets of parties.