For each power and ground pins of an MCU, it is important to use a by-pass capacitor to decouple voltage fluctuations from the rest of the circuit. This capacitor is also called a decoupling capacitor.
Because an MCU consumes very little current, even a small value capacitor like a 0.1uF or 0.22uF capacitor is suitable. At this value range, capacitors are typically a ceramic type. Ceramic capacitors natively have low ESR values. Furthermore, ceramic capacitors are not polarized.
Like by-pass capacitors of a regulator, these capacitors should be placed as close to the device as possible.