What does You can have some of this cake , but not all for it. mean?

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I'm going to make an educated guess there as I don't have much context and say that whoever wrote that is paraphrasing the English phrase "have your cake and eat it (too)". The phrase in question is used when there are two options that someone wants, but they can’t have both because the options conflict with each other, so they can only pick one.

Below is an example:

You can't have your cake and eat it - if you want more local services, you can't expect to pay less tax.

This phrase might be easier to understand if the wording was changed slightly so that it reads: “You can’t eat your cake and also have it.”

Hope this helps.