Differencing is a time series transformation used for removing unit roots. First-order differencing of a series $$x_t$$ produces a series $$\Delta x_t:=x_t-x_{t-h}$$ and removes a single unit root. Simple differencing uses $$h=1$$, seasonal differencing uses $$h=\text{# of seasons}$$. Higher-order differencing consists of consecutive applications of first-order differencing: $$\Delta^d x_t:=\Delta(\Delta^{d-1}x_t)$$ and removes multiple ($$d>1$$) unit roots.
Fractional-order differencing is also possible and is defined by $$\Delta^d x_t := x_t - d x_{t-1} + \frac{d(d-1)}{2!} x_{t-2} - \frac{d(d-1)(d-2)}{3!} x_{t-3} + \dots +(-1)^{k+1} \frac{d(d-1) \cdot \dots \cdot (d-k)}{k!} x_{t-k} + \dots$$.