Microcell corrosion is when active corrosion and the cathodic half-cell reaction that it produces take place at different points along the same piece of metal. Corrosion is, of course, an electrochemical reaction that produces voltage across an anode and a cathode. On the surface of a contiguous piece of metal, it is possible to have multiple corrosion cells, where pairs of adjacent anode-cathode electrons cover a large area of a metal surace, leading to relatively uniform corrosion.
Microcell corrosion is generally caused by concrete carbonation or high levels of chloride. This short video from CorrConnect shows how this process works at a molecular level.