To explain it in other words, the term refers to surgical weight loss procedures which are, more often than not, performed out of necessity and a lack of other options. In most cases, these procedures consist of reducing the size of the stomach or rerouting the small intestines to the small stomach pouch.
How effective is bariatric surgery?
Fortunately, a number of studies have already been conducted on the subject, and it was demonstrated that “the procedures cause significant long-term loss of weight, recovery from diabetes, improvement in cardiovascular risk factors, and a reduction in mortality of 23% from 40%”. 
An Overview of Bariatric Surgery Procedures
The types of procedures one can undergo can be separated into three main groups: malabsorptive, restrictive and combined ones. The first group contains procedures which reroute the small intestine so that a certain percentage of the food never gets absorbed by the system.