Effective stress is a concept in soil mechanics that describes the stress carried by the soil skeleton, which is crucial for understanding soil behavior. It is defined as the total stress minus the pore water pressure within the soil. This means that effective stress determines how much load the soil can support, influencing its strength and stability. The formula for effective stress is given by σ' = σ - u, where σ' is the effective stress, σ is the total stress, and u is the pore water pressure. This relationship is essential for engineers and geologists when analyzing soil conditions for construction and other projects.