In this small painting, the young Rembrandt seems to represent the daunting moments of conception and decision necessary to the creation of a work of art. An artist confronts his easel in a studio bare of everything except his essential tools. This drama, with its emphasis on thought rather than action, is intensified by the expressive use of light and shadow. The painting’s daring perspective is also important: the distant figure of the painter seems dwarfed by his work, looming large in the foreground.
(http://www.mfa.org/collections/object/artist-in-his-studio-32665)
The painting shows an artist' studio in realist style.
One critical analysis of the painting commented on the size disparity between the canvas inside the painting and the actual canvas, saying "Rembrandt's picture is small relative to its subject, rendering something far grander than itself–a painting several times its own size." (https://en.wikipedia.org/wiki/The_Artist_in_his_Studio)