A more thorough explanation:

Treasurys are debt obligations of the federal government that are backed by the full faith and credit of the government. There are different types of Treasurys, including Treasury bills, Treasury bonds, Treasury certificates, and Treasury notes. These are considered to be very safe investments because they are backed by the government.

For example, if you buy a Treasury bond, you are essentially loaning money to the government. In return, the government promises to pay you back the amount you loaned, plus interest, at a future date. This is considered a very safe investment because the government is unlikely to default on its debt obligations.