What it is and where it fits
The Siemens 6ES7951-1KL00-0AA0 is a 2 Mbyte Flash-EPROM memory card for the S7-300 PLC family, designated as a spare part for existing installations. It stores the user program and configuration data for the CPU, loading at power-up. The short design form factor fits the standard S7-300 front slot.
What the 2 Mbyte means for your program
The 2 Mbyte capacity determines how large a user program and data blocks the CPU can hold. For a typical S7-300 application — say, a conveyor line with a few hundred rungs of ladder and a handful of function blocks — 2 Mbyte is ample. But if your application uses extensive recipe data, large DBs for array processing, or multiple FB instances, check that your compiled code size fits within this limit before committing the BOM line.