What does the notation of $X_n:=n1_{(0,\frac1n)}$ in the following example mean?

Question

I don't understand the notation of the following term $$ X_n:=n1_{(0,\frac1n)} $$

It comes from this specific problem:

Let $P$ be uniform distribution on $[0,1]$ and let $X_n:=n1_{(0,\frac1n)}$ Then $X_n(w) \to 0$ for every $\omega$, but $E[X_n]=1$ for each $n$.

Answer 1

$$f(x)=n\cdot \boldsymbol 1_{(0,\frac{1}{n})}(x)=\begin{cases}n&x\in (0,\frac{1}{n})\\0& \text{otherwise}\end{cases}$$

Answer 2

$1_A$ is the so called characteristic function. It is defined by

$1_A(x) = 1$ if $x \in A$

$1_A(x) = 0$ if $x \notin A$