Learn English – Word for phrases that are examples of what they describe

The most common, and most commonly understood, term is self-reference, or self-referential if you're after an adjective.

A subset of that is autograms, "sentences that describe themselves in the sense of providing an inventory of their own characters". Examples are "This sentence has exactly six words" or "This sentence employs seven e's".

For single words rather than phrases, there's also the term autological. Textbook examples of this include "polysyllabic", "pentasyllabic", "short", "sesquipedalian", and "noun".

Lastly, there's the term iconicity that generally describes "the conceived similarity or analogy between the form of a sign (linguistic or otherwise) and its meaning". Examples include the word "bed", which some might note looks like a bed, or "looooong", made very long on purpose. Some poets, notably e e cummings, make deliberate use of visual iconicity, though auditory iconicity is more common (think onomatopoeias). Wikipedia provides this nice example of spatial iconicity:

For instance, in Cummings' grasshopper poem ("r-p-o-p-h-e-s-s-a-g-r") the word arriving begins on the far right of the poem with the "a," the "r" is near the middle of the poem, and the rest of the word is on the left of the poem. The reader must travel a great distance across the poem, therefore, in order to "arrive".