Data in transit is data that is moving from one location to another, such as over a network or through the air.
Data in transit is vulnerable to interception, modification, or theft by malicious actors. A VPN (virtual private network) is a technology that protects data in transit by creating a secure tunnel between two endpoints and encrypting the data that passes through it2.
References: CompTIA Security+ Study Guide: Exam SY0-701, 9th Edition, Chapter 4, page 145.