If you want a non-technical way to look at snapshots, check out these graphics.
Think of the “truck” as the available capacity (or pool).
Think of each “box” as a data block or file.
Think of any box with a “white sticker” as included in the active dataset/filesystem.
Think of a “color tag set” as a snapshot, which can also be viewed as a dataset/filesystem itself.
Think of a “faded” box as hidden/unavailable, and only exists in whatever snapshot(s) it is tagged with.
Read the intro and rules carefully.