What Is a zRAM in LInux ?

It's a kernel module, which provide 'swap space in RAM'. In simply words - zram creates in memory block device as a swap device. So there's no need of physical swapfile or swap partition. More, all data are compressed and reading/writing to memory is faster than on hdd/sdd, so1GB of zram is more efficient than1GB of swap partition. The only drawback is that less used pages can't be droped on physical device (and seats in memory until invalidated).