When you get in the best shape of your life in preparation for summer, more specifically for the beach.

This includes losing a lot of bodyfat to reveal toned muscle underneath as well as simply building more muscle mass.
It's a reference to Demi Lovato's hit song "Cool for the Summer." This song popularized the phrase since it was used in videos of fitness influencers working out and flexing. The phrase was popularized by fitness creators like Adam Yu and Joseph Shin.