There is nothing particularly ‘recondite’ about this little epigram; the terms Hardy employs, nature, art, measure, are pretty ordinary. But you must take care when you consult your dictionary to look for definitions relevant to the discussion of poetry; and you must keep in mind that Hardy, as a poet, maintains different senses of his words in provocative and fertile tension.
Definitions from Oxford Online Dictionaries
ART:
def 1: The expression or application of human creative skill and imagination [...] producing works to be appreciated primarily for their beauty or emotional power.
def 4: A skill at doing a specified thing, typically one acquired through practice.
NATURE:
def 1: The phenomena of the physical world collectively [...] as opposed to humans or human creations.
def 2.1: The innate or essential qualities or character of a person ...
MEASURE:
verb, def 2: Consider (one’s words or actions) carefully.
noun, def 4: The rhythm of a piece of poetry or a piece of music.
Hardy's epigram alludes to critical questions which have been debated for centuries, sometimes millennia: Does a work of art ‘imitate nature’ or ‘transform nature’ or ‘create a second nature’? Is the poet’s ability an innate gift or an acquired skill? Should poetry employ a distinct diction and meter (‘measure’) or should it emulate the lexicon and rhythms of natural speech? Is poetry a spontaneous overflow of emotion into language or is it a contrived and disciplined—‘measured’—examination of emotion?
Hardy in effect dismisses the Either/Or debates and insists that poetry is Both/And. The poet must consult his ‘natural’ (that is, authentic) emotion as it is spontaneously expressed; but he must also shape a true recreation of that emotion through his acquired ‘artistic’ skill.
That’s a lot to pack into one short sentence, and I’ve barely scratched the surface. But that’s how poets think—and linguists.