A uniform electric field has the same strength and direction at every point between the plates (ignoring edge effects). For parallel plates with potential difference V and separation d, the field strength is E = V/d. Field lines point from the positive plate to the negative plate.
A particle with charge q in this field experiences a constant force F = qE, and hence a constant acceleration a = F/m = qE/m in the field direction. Here the plates are horizontal (positive at the top, negative at the bottom), so the field is vertical downward. A charged particle launched horizontally follows a parabolic path: uniform horizontal motion and uniformly accelerated vertical motion. Neutral particles (e.g., neutrons) feel no electric force and travel straight.
The simulator uses non‑relativistic kinematics and treats the field as perfectly uniform between the plates.