It is a device-mapper target that provides transparent encryption of block devices. The term "dm" stands for device-mapper, which is a framework provided by the Linux kernel for creating virtual block devices.
With dm-crypt, you can encrypt entire block devices (such as hard drives or partitions) on-the-fly, meaning that the encryption and decryption processes are done in real-time as data is read from or written to the disk. This allows for the creation of encrypted volumes without the need to reformat the entire disk.