The phrase "intensity efforts" refers to putting in a lot of energy and focus into a particular task or activity. It means giving your best and working very hard to achieve something. Full definition
Again, I recommend you create your own warm up, finishing with high intensity effort to get your heart rate up. (hab-it.com)
Use the highest speed that will bring you up to a vigorous - intensity effort where you are breathing so hard you can only speak short phrases. (verywellfit.com)
The evidence documenting the importance of the aerobic system to repeated high intensity efforts is fairly robust. (rugbystrengthcoach.com)