The Siemens 6ES7953-8LG30-0AA0 is a Micro Memory Card (MMC) for the S7-300, C7, and ET 200 families, carrying 128 kbyte of Flash-EPROM storage. It's the program and parameter memory for the CPU — without it, the controller has no application to run. The card writes program blocks and retains data for a minimum of 10 years after final programming, with a rated endurance of at least 100,000 write/delete cycles.
The 128 kbyte capacity suits small-to-medium S7-300 applications — think a few dozen FCs and DBs for a conveyor section or a pump skid. If your project exceeds that, you'd step up to the 512 KB or 2 MB cards in the same form factor. The Flash-EPROM technology means the program survives a power loss without a backup battery, which simplifies commissioning and spares management.