Learn Dutch: Bovendien voel je je fysiek beter als je wat uitrust. - Moreover, you feel physically better if you rest a little.

The verb used here is zich voelen (to feel), which is reflexive. Because the sentence starts with the adverb bovendien (moreover), it triggers inversion, swapping the verb and the subject. The first je is the subject ("you"), and the second je is the reflexive pronoun ("yourself").

Does wat mean "what" in this sentence?

Not here. While wat translates to "what" in questions, it is also frequently used to mean "some" or "a little". Saying wat uitrust means resting just a little bit, which softens the statement and ties into the concept of minimalism.

Why is the separable verb uitrust written as a single word at the end of the sentence?

The verb is uitrusten (to rest). In a main clause, the prefix would split off (for example: je rust wat uit). However, because it is in a subordinate clause introduced by als (if), the verb gets pushed to the end of the clause and the parts remain attached.

If this lesson is about concessive minimalism, how could I use al is het maar with this sentence?

You can use it to emphasize that even a tiny amount of rest is beneficial. You could rephrase the second half of the sentence like this: ...als je uitrust, al is het maar wat (...if you rest, even if it's just a little) or ...al is het maar voor even (...even if it's just for a moment).