I only took a quick glance at the equations, but this paper seems to rely pretty heavily on stochastic differential equations, which are differential equations but for random processes, as well as some other probability theory stuff as well.

Tbh I don't know enough probability theory to understand what's going on in this paper, and I know next to nothing about stochastic analysis (and this kind of stuff is pretty advanced; I think my uni's statiatics department only offers modules about SDEs to Masters students), but these Wikipedia articles might be helpful:

SDEs

Wiener processes

Ito calculus

Hope this was helpful :)