Explain the concept of "bearing capacity" in soil mechanics and its significance in foundation design. Discuss the factors influencing the bearing capacity of soil, including soil type, moisture content, and depth of the foundation. Additionally, elaborate on the different methods used to determine bearing capacity, such as the Terzaghi's bearing capacity equation and field tests like standard penetration test (SPT). How does understanding bearing capacity contribute to ensuring the stability and safety of structures built on soil foundations?