Store data in flash (program) memory instead of SRAM. There's a description of the various types of memory available on an Arduino board.>
The PROGMEM keyword is a variable modifier, it should be used only with the datatypes defined in pgmspace.h. It tells the compiler "put this information into flash memory", instead of into SRAM, where it would normally go.
PROGMEM is part of the pgmspace.h library that is available in the AVR architecture only. So you first need to include the library at the top your sketch, like this: