RingCT is how transaction amounts are hidden in Monero.
These confidential transactions can include cryptographic proof that the sum of the input amounts is the same as the sum of the output amount, without actually revealing the numbers themselves.
This means that you are still able to trade which address sent and received cryptocurrency.
RingCT manage to protect the privacy of more than just the transaction amounts, however. For example, Monero transactions are untraceable due to ring signatures and the destination of funds is obscured thanks to stealth addresses.
RingCT was implemented in block #1220516 in January 2017, and after September of the same year, this feature ended up becoming mandatory throughout all of the transactions which were conducted on the network itself.
RingCT introduces an improved version of ring signatures known as a multi-layered linkable spontaneous anonymous group signature, which allows for hidden amounts, origins as well as destinations of transactions with a high level of efficiency and is a verifiable as well as trustless coin generation method.