Both stable and secure with interest rates usually slightly higher than Tbills, short term bonds are a common substitute for portfolio cash.
Short term bond funds generally have an average maturity around 2-3 years. The returns on the site use a 3-1 model that mirrors popular Bloomberg Barclays index methodology. They are based on the highest quality bonds issued by the treasury of the country in question.
Different countries have different names for treasury bonds. In the context of this site, treasuries, gilts, and bunds are all interchangeable.