What is meant by limit of sets?

When all you have is the raw set structure, the only limit concept that really makes sense is:

$S$ is the limit of the sequence $S_1, S_2, S_3,\ldots$ iff $$ \forall x\; \exists N\in\mathbb N\; \forall n>N : x\in S\Leftrightarrow x \in S_n $$

In other words every possible element is either in all but finitely many $S_n$ (in which case it is in the limit set too), or in only finitely many $S_n$ (in which case it is not in the limit set). If there is even one $x$ that is both present in infinitely many of the $S_n$ and absent from infinitely many of them, then the sequence does not have a limit.

This notion corresponds to a pointwise limit of indicator functions.

If the sequence of sets is increasing, then the limit is simply the union of all the sets. If it is decreasing then the limit is the intersection of all the sets.

We have (see e. g. Billingsley, Probiability and measure, 3rd edition, page 52), that the limes superior and the limes inferior are defined - as for any ordered space - on the power set:

Definition. Let $X$ be a set, $(S_n)$ a sequence in $\mathfrak P(X)$. We define
(1) The limes superior of $(S_n)$ is the set which contains all elements which are frequently in $S_n$, that is in infinitely many $S_n$: $$ \limsup S_n := \bigcap_{n\ge 1} \bigcup_{k\ge n} S_k $$ (2) The limes inferior of $(S_n)$ is the set which contains all elements which are finally in $S_n$, that is in all but finitely many $S_n$: $$ \liminf S_n := \bigcup_{n\ge 1} \bigcap_{k\ge n} S_k $$ (3) If $\liminf S_n$ and $\liminf S_n$ coincide, we say, $(S_n)$ converges and write $$ \lim S_n := \limsup S_n = \liminf S_n. $$

Example: If $S_n$ is montonically increasing, then $\bigcup_{k\ge n} S_k$ is equal for all $n$, hence $\limsup S_n = \bigcup_{k\ge 1} S_k$, on the other side, $\bigcap_{k\ge n} S_k = S_n$, hence $\liminf S_n = \bigcup_{n\ge 1} S_n$, that is $\lim S_n = \bigcup_n S_n$.