When we make a diode, there is a region where the p type and n type materials interact and that is known as junction. This junction offers resistance to the current flow which contributes to junction resistance. So it is the resistance offered by the p-n junction. Well as you know a diode can be forward or reverse biased. Thus there are 2 types of junction resistance. 1. Forward biased junction resistance 2. Reverse biased junction resistance.
In forward bias state , the resistance could be classified into static(DC) and dynamic (AC) based on the nature of current flow. In reverse bias state, an idel diode offers infinite resistance. But in practical case, there exists a small leakage current due to the minority thermal carriers. And hence it is less than infinty. The resistance offered by the junction in the reverse biased condition is much greater than the resistance in forward biased.
Series resistance is the sum of resistance values of the individual resistors connected serially.