Programming in tm4c123gxl evaluation kit

I am using tm4c123gxl evaluation kit. I saw the following lines in a c-program (written for the microcontroller on that board). Can anyone tell me the meaning of the following lines.

#define SYSCTL_RCGC2_R          (*((volatile unsigned long *)0x400FE108))

#define SYSCTL_RCGC2_GPIOF      0x00000020  // port F Clock Gating Control

SYSCTL_RCGC2_R |= 0x00000020;     // 1) F clock

delay = SYSCTL_RCGC2_R;           // delay   

What it means by activating clock for a port.