You will see similar references in articles that discuss politics, most generally when the person, like Kamala Harris, is a Senator. The first letter, D, refers to her political party- in this case, Democrat. R refers to Republican, and I refers to an independent- someone who has no political affiliation. The second two letters are an abbreviation of the state the senator represents. In this case, California. Similarly, you could see NY for New York, FL for Florida, TX for Texas, etc.
I say generally for senators because in the American political system, there are two chambers of Congress, the Senate, and the House of Representatives. Each state has two senators which represent the entirety of the state, so one will typically see the designation after their name that you cite. However, the House of Representatives has over 400 members, and how many members depends on the population of each state. So larger states like California will have more Representatives than less populated states like Wyoming. Further, Representatives are voted in by localized districts, and represent that district, not the entire state like Senators do. Therefore, in most news articles the designation after a name is omitted, and they are referred to as Rep. X, with party and state or district written out somewhere else. For example, Rep. Alexandria Ocasio-Cortez, democrat from New York. However, the designation could be included, like you cite, and will again first include a letter representing policy, followed by the state or district they represent. If the district is cited, it is usually the state followed by a district number. So again using Rep. Ocasio-Cortez, it could look like "Rep. Ocasio-Cortez (D- NY-14)" or something similar.