The Raspberry Pi family typically boots from an SD card plugged to the system. To avoid SD card lifetime issues, these systems can run from an connected USB resp. USB disk stick or – on some Pi derivatives – from an onboard eMMC flash memory.
Lifetime issues may occur from the fact that the number of write cycles to SD cards are much lower than to USB sticks/disks or eMMC. Additionally, systems often boot quicker from USB resp. eMMC compared to a plugged SD card.
Photos of SBCs with different block devices attached
At best, you can start with the USB device resp. a removable eMMC card) by flashing DietPi directly to this.
In case of an already running system, there may be the need of moving this running system to a different location: This blog describes how to move your system to a USB stick/disk or to the onboard eMMC.