A "Series" is an investment Round where all the investors invest on the same set of terms, such as price, dividend and liquidation preference rights etc. Often, investors present their investments at approximately the same time, and the Round "closes" on a fixed date for all investors. The Series letters are just in order of the investment rounds, the first Round being Series A, the second round being Series B, third round being Series C, fourth round being Series D, and so on.