White-collar jobs were jobs that had people working in an office, this started becoming more and more prevalent after World War II. Blue-collar jobs were jobs that usually had people doing manual labor, like working in a factory for example.
After World War II and even during the Cold War, technology was advancing and becoming more widespread in facilities and workplaces. More advanced and automated technology was becoming more common in factories, which caused many blue-collar workers to be out of jobs and they needed to find something elsewhere.
Since things were becoming more automated and advanced, the jobs in factories were dwindling down, while office work was expanding. Many of the blue-collar workers that were losing their jobs ended up working in the office and becoming white-collar workers.