In the United States, the two major political parties are the Republican Party and the Democratic Party.
Have only two major parties means that both parties have a pretty big "tent" in terms of what views are represented in both parties.
Unlike countries with a parliamentary system and many parties represented, America's two parties forces disparate groups to come together and find common ground to elect preferred candidates.
This means, though, that every now and then groups break off.
The last major group to break off were the Southern Democrats who broke off and joined the Republicans because of Civil Rights.
So, for a person interested in running for office, their first step is to decide which party better represents their broad views. After that, the party will work to elect them and that might involve changing some views to represent the party's overall view.