Kernel address space layout randomization, or KASLR, is a defense that loads the operating system kernel at a randomly chosen memory address each time the system boots. Many exploits need to know where particular kernel code or data sits in memory, and randomizing the location makes it harder for an attacker to aim at them reliably.
KASLR raises the difficulty of exploitation but does not prevent it. If an attacker can find or leak a kernel address, for example through an information-disclosure bug, the randomization can be bypassed. Some kinds of attack do not depend on knowing addresses at all. It is therefore best viewed as one layer of defense among several.
Because of this, KASLR does not make a vulnerable kernel safe. A flaw that can be triggered and exploited remains a flaw even when randomization is on, and a patch is still the actual fix. Where KASLR is enabled also depends on the kernel configuration and the distribution.
To make the most of it, keep the kernel updated, leave default hardening options enabled, and combine it with other protections such as limiting access to kernel information and running services with the least privilege. Treat exploit reports that claim to defeat KASLR as a reason to prioritize patching, not as a reason to disable it.