The Church is obeying Jesus's command to build temples all around the world. These sacred buildings are literally houses of God where Jesus may visit. They are also holy places of worship where certain rituals must be performed in order for humans to return to the presence of Heavenly Father in the eternal afterlife and eventually become gods themselves. To enter a temple requires a current temple recommend.
To receive a temple recommend, you must be a holder of the Melchizedek priesthood if you are male, or a member of the Church for at least a full year if you are female. You will be interviewed by two Church leaders who will ask you questions to determine your worthiness, such as: "Are you paying a full tithe?", "Do you obey the law of chastity?" If all goes well, you will receive a signed temple recommend that remains valid for two years.
First among the essential rituals that must be performed inside a temple is the endowment ceremony. Males receive their endowment just before serving their two-year full-time mission or before getting married. Females receive the endowment before they get married or at their bishop's discretion. After you enter the temple, you will change into white temple clothing. During the endowment ritual, you will receive symbolic teachings about Creation, Adam and Eve, the apostasy, and the restoration of the gospel.
The temple ceremony was designed by Heavenly Father himself. You will make a series of solemn promises to God. You will also be taught all the signs, keywords, and tokens that will be necessary after your death to pass by the angels who stand as sentinels, allowing you to walk back to the presence of Heavenly Father, and gain your eternal exaltation, eventually becoming a god yourself.
You will be given special sacred underclothing known as a temple garment that you will solemnly promise to wear day and night for the rest of your life. The garment should be removed only when absolutely necessary and then worn again as soon as possible. It will act as a reminder of the promises you've made to God in the temple, and as long as you remain faithful to your promises, it will act as a protective covering allowing you to withstand the fiery darts thrown by Satan and his angels.