The lesson focuses on developing a word count function in Go and addresses challenges related to accurately counting words in various input scenarios. Initially, the function mistakenly counts spaces instead of words, leading to incorrect results, particularly when handling multiple spaces and newline characters. The lesson introduces a solution by leveraging the Go standard library's `strings.Fields` function to simplify word counting, which efficiently splits strings around whitespace characters. The instructor emphasizes the importance of utilizing existing library functions to improve code readability and maintainability. Additionally, the lesson transitions towards best practices in testing by suggesting the separation of test code from application code, enhancing the approach to black-box testing. Finally, it hints at future enhancements, including error handling and expanding functionalities for a comprehensive command-line application.