It is a "one factor model" as it describes interest rate movements as driven by only one source of market risk.
The model specifies that the instantaneous interest rate follows a geometric Brownian motion: where Wt is a Wiener process modelling the random market risk factor.
This is one of the early models of the short-term interest rates, using the same stochastic process as the one already used to describe the dynamics of the underlying price in stock options.
Its main disadvantage is that it does not capture the mean reversion of interest rates (their tendency to revert toward some value or range of values rather than wander without bounds in either direction).
Note that in 1979 Rendleman-Bartter also published a version of the Binomial options pricing model for equity underlyings.