1. The problem is to understand the interval $[-1,3]$ and what it represents. 2. The interval $[-1,3]$ is a set of all real numbers $x$ such that $-1 \leq x \leq 3$. 3. This means the interval includes all numbers starting from $-1$ up to $3$, including the endpoints $-1$ and $3$. 4. Intervals are often used to describe domains or ranges of functions or solutions to inequalities. 5. For example, if a function $f(x)$ is defined on $[-1,3]$, it means $f(x)$ is defined for every $x$ in that range. 6. The square brackets $[\ ]$ indicate that the endpoints are included (closed interval). 7. If the interval were $(-1,3)$ with parentheses, it would mean the endpoints are not included (open interval). 8. In summary, $[-1,3]$ is the set $\{x \in \mathbb{R} : -1 \leq x \leq 3\}$, including all numbers between and including $-1$ and $3$.