This is literally me making things up for the most part based on what I've seen in fate/zero and in the game fate grand order.

*large inhale*

From what I've seen, a reality marble is an alternate reality within a reality. It can take the place of little pocket dimensions that certain servants use for their noble phantasms or in nursery rhyme's case act as the means to their existence.

It's a very high level of magic to be able to cast such spells, and due to the large amount of mana (magical energy) that one has to spend, they're usually reserved for a servant's noble phantasm. Some examples of that?

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Iskander's reality marble noble phantasm contains a vast empty desert, in addition to a MASSIVE army.

This is used like three times in the entire show; it costs a large amount of power.

Nursery Rhyme is a tad different; there's versions of her where she was able to send her enemies into pocket dimensions containing scenes and characters from the original Alice in wonderland.

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Honestly this video is a much proper explanation not only of the servant as a whole but also why a servant who takes mostly takes the form of a young child is so terrifyingly powerful

I love her she's precious but yikes.

.....

Anyway, I'll find what other servants use reality marbles as noble phantasms/or as their way of forming a body, so uh watch out for that