You have selected the following FlashKernel for Mobile ODIN:

Samsung Galaxy S3 LTE GT-I9305
v5.0, based on MobileODIN Recovery v1.35

After clicking download, your device should start downloading the file. When the download is complete, you need to open the file to start installation.

FlashKernels are special kernels used by Mobile ODIN to help flashing the selected firmware, if it cannot be done with the kernel you are currently running. The FlashKernel is used temporarily, it does not remain on the device after the firmware flash is complete.