Apart from an academic environment (universities) he, all alone, succeeded in a rather short time to shake up the then current fundamental understanding of physics and to open up far reaching scientific perspectives. In view of his 48 hour per week work load at the Office, this feat is really astounding. There is to add that during that same time period, Einstein and his family travelled abroad to Zagreb and Novi Sad to visit Mileva's family.
In the first of these papers, addressing "Energetic Properties of Light", he argued that there must be a fundamental difference between the description of the propagation of light and of its interaction with matter, respectively. He emphasized the discrepancy between the well established propagation of light as a travelling electromagnetic wave and, as he concluded from considering the thermodynamic properties of radiation, the obtruding description of light emission and -absorption in the form of quantized energy transfers between radiation and matter. Of course, he recognized the emerging fundamental problem which bothered him continuously, but he could not find a satisfactory solution for many years to come. It was, however, this work which finally earned him the Nobel prize in 1921.
In the second publication Einstein attempted to determine the size of molecules or atoms, respectively, by analyzing diffusion phenomena in dilute solutions and their relation to internal friction. He submitted this essay, directly proving the molecular structure of materials, as his doctoral dissertation to the University of Zürich and dedicated it to his friend Marcel Grossmann.
With the third publication, Einstein succeeded in offering an understanding of the long and well known but yet unexplained Brownian motion, i.e., the thermally induced motion of suspended particles in liquids at rest. From his discussion, involving the theory of heat, it follows that with an optical microscope it is possible to experimentally determine Avogadro's number, i.e., the number of atoms contained in a unit volume. A really surprising result.
Most attention on short terms generated the fourth paper on "Electrodynamics of moving bodies" which he later, after the advent of the General Theory of Relativity 1915, renamed as the "Special Theory of Relativity". The insight that the state of absolute rest cannot be verified by experiment led Einstein to formulate the so-called "principle of relativity" and postulated it to be universally valid. With the additional assumption that the speed of light in vacuum is fixed, i.e., the speed of light is independent of the velocity of a moving light source, he, by applying kinematics, succeeded in finding new relations between space- and time coordinates which are, to a large extent, counterintuitive.
In the last of the five mentioned publications of that short period of time, Einstein described a consequence of his Theory of Relativity and showed that energy and mass are equivalent as formally is captured in the famous equation E = mc2. To be precise, here E denotes the ‘rest energy E0’. Note that the mass of a body is independent of its motion.