From the word itself, Suboptimal means the route that the packet takes is not the most optimal or in other words not efficient of all the possible routes.
Suboptimal routing can be present in a lot of forms. So i’ll just give an example. let’s take 3 routers in a triangle topology.
R1–directly connected to LAN1
R3–directly connected to LAN3
From this example, the optimal route from LAN1 to LAN3 is from R1-R3 and vice versa.
Sub-optimal route is R1-R2-R3 path and vice versa. There may also be a case wherein outgoing traffic from R1 it takes the optimal route(R1-R3) but the return route(incoming traffic) is suboptimal(R3-R2-R1 path).