It is a term coined in psychology that means thinking about our thinking. It is a skill that forms foundation for self-awareness and regulation.
One tool that can help in the journey of self-awareness and regulation is writing down our thoughts/ journaling.
It doesn't mean that tou have to write beautiful narrative or become a writer. It simply means you spend some time writing your own thoughts. It could be phrases, broken sentences, or just words.
Thus exercise helps give words to your thoughts and the process leads to awareness. Our hidden thoughts become more obvious and we become more aware of our biases, beliefs and we get closer to making the positive changes.