According to Paul, he wants to tell people about Jesus in new places where no one else has talked about Him yet, and he doesn't want to brag about things other people have done. He wants to do his own work and help more people learn about Jesus.
2 Corinthians 10:16: To preach the gospel in the [regions] beyond you, [and] not to boast in another man's line of things made ready to our hand. [line: or, rule]
This verse emphasizes not preaching where Christ was already named, whereas 2 Corinthians 10:16 discusses preaching in areas beyond existing influence.
Romans 15:20: Yea, so have I strived to preach the gospel, not where Christ was named, lest I should build upon another man's foundation: