Diego Garcia is an island in the Indian Ocean located roughly halfway between Singapore and Dar es Salaam. This article and another similar one suggest US long-range bombing assets are massing at Diego Garcia. The means one of two things.
Either they are there to add weight to the pressure Trump will put on Iran to reach a no-nukes deal, or they are there to bring pressure and, if the diplomatic effort fails, bomb to destruction Iran's nuclear program using (non-nuclear) ground-penetrating high explosive bombs. Carrier based assets will aid in this latter effort.
The intelligence behind this article comes from open-source unclassified civilian satellite look-down video. There is not a lot of overhead privacy in this modern age.
