modular calculation

Question

$$21 \equiv 4\pmod{17} \equiv \ ?$$

What does this expression mean? I know what mod mean, and what $\equiv$ means, but together this equation doesn't seems meaning anything. The $21\equiv 4 \mod{17}$ makes sense, but what's the succeeding part ?

Answer 1

$21\equiv4\bmod17$ means $17|21-4$. In fact, $21\equiv4+17k\bmod17$ for all $k\in \mathbb Z$.

In particular, taking $k=2$, we have $21\equiv38\bmod17$.

So $38$ is an answer, but there are infinitely many acceptable others, including $55$ for example.