This is a custom flash cartridge that was designed from scratch, that supports RTC just like a retail RTC game. The best part is the 32KB of FRAM, so your save file will not be lost when the battery dies.
A flash cartridge is just a normal cartridge that can be re-programmed. When you buy a retail cartridge, it will always contain the same game that it had when you bought it, and cannot be changed. A flash cartridge can be changed to any other game you want, as long as you have enough memory to hold it. Tired of playing Mega Man? Just overwrite it with Pokemon Silver!
RTC stands for 'Real-Time Clock'. Not many games use a real-time clock, but the games that do use it have several time-based features that are inaccessible without it.
In a normal Gameboy cartridge, there is a type of RAM called SRAM, or Static RAM. This RAM is just extra memory storage that can be used for many things. If your game has a save feature, your save file will be stored in RAM. The problem is, SRAM needs a constant power supply to retain the data stored within. You may have noticed that cartridges containing SRAM also contain a coin battery to power the SRAM. Have you ever started up a game and noticed your save file was gone, or maybe you tried to save your game, and it was gone the next time you tried to play? This is likely caused by a dead battery in your cartridge. FRAM, on the other hand, does not require a constant power supply. If FRAM is used instead, the battery can be removed and the save file will remain intact. However, the RTC still requires the battery to keep track of time. When the battery dies, one can just replace the battery and reset the clock without having to worry about losing game progress. This is especially useful in games such as Pokemon, where players tend to invest many valuable hours.