The Riemann-Liouville fractional derivative of order $\alpha$ of a continuous function $f:(0,\infty)\rightarrow \mathbb R$ is defined as: $$D^{\alpha}=\frac{1}{\Gamma(n-\alpha)}(\frac{\partial }{\partial t})^n\int_{0}^{t}(t-s)^{n-\alpha -1}f(s)ds$$ where $n=[\alpha]+1$.
We know that the first derivative interpretes the function's slope, and the second interpretes its concavity. What is R-L fractional derivative intetrpretation?