Diodes are electronic devices that tend to allow current flow in one direction (forward biasing) but resist current flow in the opposite direction (reverse biasing). Semiconductor diodes are made of two layers: a P layer and an N layer. Semiconductors are almost pure crystal like structures that have been "doped" with a trace amount of materials that make them either Positive (P type) or Negative (N type). The N type semiconductor have a few extra electrons floating around and the P type have a few holes (places where electrons are needed) floating around. When voltage is applied in one direction (forward biasing) the electrons and holes are pushed together and current flows. When voltage is applied in the other direction (reverse biasing) the electrons and holes are driven away from the PN junction and no current flows.
PIN diodes are like ordinary PN diodes except that they have an extra layer between the P and N layers. The central layer is not doped so it is neither N nor P. PIN diodes have lots of applications and for some applications only the P and I layers are layers of semiconductors and the N is just a small dot on the I layer. For some applications only the I layer is normal sized and both the P and N "layers" are dots on the I layer.
PIN diodes behave just like PN diodes under reverse biasing: no current flows. Under a small forward biasing, the electrons and holes are driven into the I layer but no current flows. When the forward biasing is large enough, the I layer becomes saturated and the electrons and holes combine and current flows.
At Radio Frequencies (RF) and microwave frequencies (uF), PIN diodes can "breakdown" (allow reverse current to flow) at exactly proscribed frequencies--determined by the relative size of the I type center level. This means that PIN diodes can act as frequency detectors--to pass desired frequencies or shunt undesired frequency. The lag time of currents in the I level means that PIN diodes also can act a "phase shifters" to shift the angular relationship between the voltage and current of a signal. They are the only practical way to phase shift microwave signals.