A running mate is a person running for a subordinate position on a joint ticket during an election.
The term is usually used in reference to a prospective vice president of the United States, who the prospective president appoints sometime during the course of the election campaign. The term is also applied in US states where the governor and lieutenant governor run together on the same ticket.
In American presidential elections, the vice-presidential candidate is chosen by the presidential candidate. In some states, the gubernatorial candidate chooses his/her running mate while in other states parties hold separate ballots for governor and lieutenant governor. The two then run together as running mates in the general election.
In some states, separate elections are held for governor and lieutenant governor. In this case, the governor and lieutenant governor run separate campaigns and can be from two different political parties. This was the case when George W. Bush was Governor of Texas. His first lieutenant governor, Bob Bullock, was a Democrat. In cases like this, the governor and lieutenant governor are not considered running mates because they are not elected on the same ticket.
Running mates are often chosen to balance the ticket. That is to create more widespread appeal for a ticket by expanding it appeal across geographic or ideological lines. This was especially true in presidential elections before the American Civil War when northern candidates were often paired with southern candidates and vice versa.
Electoral votes also play a part in modern presidential running mate selection. Vice-Presidential candidates are often from populous states with a large number of electoral votes that may be swayed by having a favorite son on the ticket.
https://en.wikipedia.org/wiki/Running_mate
