The velocity chart typically shows the amount of work completed over time. If the "Actual Effort" line is below the "Planned Timeline," it indicates that the project is not progressing as fast as planned, hence it is behind schedule. This is inferred by comparing the actual work completed against what was scheduled.
References = The answer is based on standard project management practices and the typical interpretation of velocity charts in project management. For detailed information, please refer to the CompTIA Project+ Study Guide and other official CompTIA resources.