Number sense refers to an individual's ability to understand and work with numbers, including basic arithmetic operations such as addition, subtraction, multiplication, and division. It also involves understanding concepts like fractions, percentages, ratios, and proportions. People with strong number sense are able to estimate quantities, make quick calculations in their head, and recognize patterns or relationships between numbers. Developing a good number sense is important for success in mathematics and can be improved through practice and exposure to various mathematical concepts.