Croquis is a fancy French word that means "sketch" or "drawing." It's sort of like when you draw pictures in your coloring book, but instead of coloring inside the lines, you draw fast and loose to capture the movement and energy of what you're seeing.
Croquis drawings are often used in fashion design and figure drawing classes to capture the fluidity of a model's movements. Imagine you're at a fashion show and you see a model walking down the runway. You might take out a notebook and quickly sketch what you see, trying to capture how the fabric of the clothing moves as the model walks.
People who practice croquis often use quick, gestural strokes to create their drawings, instead of focusing on details. The idea isn't to make a perfect drawing, but to capture the feeling of what you're seeing in the moment.