This verse is a small piece of a travel narrative in the book of Acts, detailing Paul's third missionary journey. To understand its meaning, let's break it down:

"When he met us at Assos..."
"He" refers to Paul.
"Us" refers to Luke (the author of Acts) and Paul's traveling companions (likely including Timothy, Sopater, Aristarchus, Secundus, Gaius, and Tychicus - mentioned in the verses just before).
"Assos" was a port city on the coast of Asia Minor (modern-day Turkey). Paul chose to walk to Assos, while his companions sailed there directly from Troas (Acts 20:13). This might have been for personal reasons – a chance for reflection or to deal with something privately.

"...we took him aboard..."
This refers to the ship that Luke and the companions were sailing on. They picked Paul up in Assos as planned.

"...and came to Mitylene."
"Mitylene" was the capital city of the island of Lesbos, located just off the coast of Asia Minor. This was their next stop on their journey.

In essence, the verse simply describes a part of Paul's itinerary: Paul joined his companions in Assos, they boarded their ship, and then sailed to Mitylene.

Significance in the overall narrative:

It highlights the meticulous planning of Paul's journey. The meeting at Assos demonstrates a deliberate arrangement.
It emphasizes the teamwork and companionship involved in Paul's ministry. He wasn't traveling alone but with a dedicated group of people.
It underscores the geographical scope of Paul's missionary work. This small verse gives a glimpse into the many cities and regions he was traversing.
It sets the stage for the events that follow. Mitylene was just one stop on the way to Jerusalem, where Paul anticipated facing persecution.

While this verse isn't filled with deep theological meaning, it provides important context to the broader story of Paul's travels and the spread of the gospel in the early church. It's a reminder of the practical realities of missionary work, involving travel, coordination, and a supportive team.