Hexadecimal numbers are numbers represented in base $16$ , which means the digits instead of just $0$ - $9$ .
Just as in base $10$ , we have the $1$ s place, $10$ s place, $100$ s place, $1000$ s place, $10,000$ s place, etc. (the powers of $10$ ), in base $16$ we have the $1$ s place, $16$ s place, $256$ s place, $4096$ s place, etc. (the powers of $16$ ).
In base $10$ , the number $13$ means one group of $10$ and $3$ ones.  In base $16$ , the number $13$ means one group of $16$ and $3$ ones.  (This would be equivalent to $19$ in base $10$ .)
Below we count up to $32$ in base $16$ :
 BASE $16$ BASE $10$ $0$ $0$ $1$ $1$ $2$ $2$ $3$ $3$ $4$ $4$ $5$ $5$ $6$ $6$ $7$ $7$ $8$ $8$ $9$ $9$ $\text{A}$ $10$ $\text{B}$ $11$ $\text{C}$ $12$ $\text{D}$ $13$ $\text{E}$ $14$ $\text{F}$ $15$ $10$ $16$ $11$ $17$ $12$ $18$ $13$ $19$ $14$ $20$ $15$ $21$ $16$ $22$ $17$ $23$ $18$ $24$ $19$ $25$ $1\text{A}$ $26$ $1\text{B}$ $27$ $1\text{C}$ $28$ $1\text{D}$ $29$ $1\text{E}$ $30$ $1\text{F}$ $31$ $20$ $32$