From the Church Newsroom:
Latter-day Saints wear ceremonial clothing inside temples as part of the most sacred ordinances, or ceremonies, of their faith. The First Presidency has recently made adjustments to the clothing to be more simple, comfortable, and cost-effective, as well as to enhance the temple experience for patrons. This change does not alter the sacred nature of covenants made in the temple. Ceremonial clothing previously used may still be worn for temple worship. Below is the letter that is linked above.