According to the expectations hypothesis, the shape of the yield curve results from the interest rate expectations of market participants. More specifically, it holds that any long-term interest rate simply represents the geometric mean of current and future 1-year interest rates expected to prevail over the maturity of the issue. The expectations theory can explain any shape of yield curve.
Expectations for rising short-term rates in the future cause a rising (upward-sloping) yield curve; expectations for falling short-term rates in the future will cause long-term rates to lie below current short-term rates, and the yield curve will decline (or slope downward).
Thus, an upward-sloping yield curve implies that interest rates are expected to increase in the future.