The claim to fame that these two men have is that they were the first African-Americans to serve in the United States Senate. They both did so in the Reconstruction Era, which came right after the Civil War. After them, the next African-American to be elected to the Senate was not elected until 1967. There have only been three other African-Americans who have ever been elected to the US Senate (three others have served in the Senate after being appointed to fill vacant seats).
In the Reconstruction Era, the United States government took control of the states in the South. The Northerners enforced laws that allowed African Americans to vote for the first time. They also created state legislatures that were filled with Republicans. At this time, US Senators were not elected by a popular vote. Instead, they were selected by state legislatures. Therefore, both Revels and Bruce were elected not by the people but by the state legislatures.
Revels has the distinction of being the first African-American to serve in the Senate when he was elected to serve out the term for a seat that had been vacant since Jefferson Davis vacated it at the time of secession. Revels served only about one year. Bruce, by contrast, was elected in his own right and served a full term.