Meaning of the phrase "$X'$ can be identified with a subspace of $X$"?

Question

The following is taken from

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$\textbf{Exercise}$ Show that if a sequence

$$0\xrightarrow{}X'\xrightarrow{}X\xrightarrow{}X''\xrightarrow{}0$$

is exact, then $X'$ can be identified with a subspace of $X$ and $X''$ with $X/X'.$

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For the above exercise, what does it mean by "$X'$ can be identified with a subspace of $X$?"

Thank you in advance