I cannot figure out the meaning of the negative sign in the following examples, where $\mathcal({F}_t)$ is a filtration,$(M_t)$ is $\mathcal({F}_t)$-martingale $(M_t)$ (or simply a random process), a stopping time T.
i) $\mathcal({F}_{0-}) = \mathcal({F}_0)$
ii) $M_{t-}$ and $M_{T-}$
That notation means a time right before the index in question, a usual language for a limit. In other words, if you are saying $F_{t-} =F_t$, you really mean to say that $$ \lim_{s \to t^-} F_s = F_t, $$ or alternatively, that $F_t$ is left-continuous.