Vector addition is a mathematical operation that combines two or more vectors to produce a resultant vector. A vector is defined by both a magnitude (size) and a direction. When adding vectors, you can use the head-to-tail method, where the tail of one vector is placed at the head of another, or you can break them down into their components and add the corresponding components together. The resultant vector represents the overall effect of the combined vectors. For example, if you have a vector representing a force of 10 N to the right and another vector representing a force of 5 N upward, the resultant vector can be found using the Pythagorean theorem, resulting in a new vector that shows the total force acting at an angle.