Physical Address Extension or  PAE as it is generally called, is a technology which allows 32 bit operating systems to use up to 64 Gb of memory (RAM), something which is normally achieved by switching to a 64 bit system.

Read more about it HERE

If your Linux box has 4GB RAM or above, you need to install PAE kernel

On RHEL / CentOS

yum install kernel-PAE


Ubuntu 10.04 – Lucid Lynx and newer iterations automatically installs the PAE kernel if it detects more than 3G of memory. If you need to install it manually.

sudo aptitude install linux-generic-pae linux-headers-generic-pae

Once installed ensure that the grub has the pae kernel as the default and then reboot your machine.