SystemTap provides environment to get more information on almost any component within the kernel so that it may be further analysed. OR SystemTap is tool which provides facility to user to study and monitor the activities of the kernel in detail.
To run systemstap user need root privileges, not all users can run systemstap. To allow secondary users to run SystemTap without root access need to add them toone of these user groups : "stapdev" , "stapusr"
In short, When we run systemtap, systemtap builds a kernel module out of that script and then loads the module into the kernel. Once this done kerenl allow it to extract specified data directly from kernel.
To get more information on system tap refer below link : http://sourceware.org/systemtap/langref/SystemTap_overview.html