The electroweak phase transition refers to a process in the early universe where the fundamental forces of electromagnetism and the weak nuclear force unified into a single force. This transition occurred as the universe cooled after the Big Bang, leading to the formation of particles like W and Z bosons, which mediate the weak force. During this phase transition, the symmetry between these forces was broken, resulting in distinct interactions. This event is crucial for understanding the conditions that led to the formation of matter and the structure of the universe as we know it today.