"To break the tension" means to relieve or reduce a feeling of physical or emotional strain or stress in a situation. It refers to doing something that lightens the atmosphere or eases an uncomfortable or serious mood. Full definition
There were also moments of humor to break the tension of the mature themes of the movie. (fatmovieguy.com)
Taking a little time off to break the tension always helps — you'll probably come back refreshed. (sciencemag.org)
However, the wise - cracking sock monkey breaks any tension this movie might have built in terms of a horror or thriller story. (fatguysatthemovies.com)