“crashkernel=X@Y” is a kernel parameter for Kdump.

Kdump requires some memory reservation for the second kernel(capture kernel).

When kdump is enabled, (physical memory – X) will be allocated.

Kdump is disabled:

Example:
# uname -r
2.6.18-8.1.15.el5PAE
# cat /proc/cmdline
ro root=/dev/VolGroup00/LogVol00 rhgb quiet
# free
total used free shared buffers cached
Mem: 3764892 522644 3242248 0 32436 346112
-/+ buffers/cache: 144096 3620796
Swap: 2031608 0 2031608

Kdump is enabled:

Example:
# uname -r
2.6.18-8.1.15.el5PAE
# cat /proc/cmdline
ro root=/dev/VolGroup00/LogVol00 rhgb quiet crashkernel=512M@16M
# free
total used free shared buffers cached
Mem: 3240604 3091352 149252 0 17840 2663684
-/+ buffers/cache: 409828 2830776
Swap: 2031608 116 2031492

X: denotes how much memory to reserve for the second kernel(capture kernel)

Y: denotes what physical address the reserved memory section starts.

X-Y=Z: Capture kernel runs form reserved memory location(Z)