Jesus is the Christ

What is meant by the testimony, "Jesus is the Christ" (Acts 18:5)?
χριστον (Christon), the original Greek word translated "Christ" in "Jesus is the Christ," literally means the "Anointed One" and refers to the One anointed to be the Savior as per John 3:16.

Was the arrival of Silas and Timothy the first time Paul "testified to the Jews [of Corinth] that Jesus is the Christ" (Acts 18:5)?
Paul could not have preached the Gospel, let alone "persuaded" (Acts 18:4) anyone about the Gospel without testifying that Jesus is the Christ.

Then why is the fact that he testified Jesus is the Christ restated here?
It was his final call to the Jews of Corinth. Paul had been witnessing to and sharing the Gospel to the Jews of Corinth until Silas and Timothy arrived. The Holy Spirit used their arrival to turn Paul's ministry toward the "gentiles" (Acts 18:6) of Corinth and had "compelled" (Acts 18:5) Paul to make a final declaration to the Jews so that those who had been converted could leave with him.

Weren't "Silas and Timothy" supposed to meet Paul in Athens?
Yes - "So those who conducted Paul brought him to Athens; and receiving a command for Silas and Timothy to come to him with all speed, they departed" (Acts 17:15) - and they did meet Paul in Athens. From Athens, Timothy was dispatched back to Thessalonica to minister to the young church there - "Therefore, when we could no longer endure it, we thought it good to be left in Athens alone, and sent Timothy, our brother and minister of God, and our fellow laborer in the Gospel of Christ, to establish you and encourage you concerning your faith, that no one should be shaken by these afflictions; for you yourselves know that we are appointed to this" (1 Thessalonians 3:1-3) - and Silas apparently to another region of "Macedonia" (Acts 18:5). And from Macedonia, Silas and Timothy rejoined Paul in Corinth, bringing financial support from the Macedonian Christians as Paul later indicated to the Corinthian church: "And when I was present with you and in need, I was a burden to no one, for what I lacked the brethren who came from Macedonia supplied..." (2 Corinthians 11:9).