What does it mean if a catalyst support is doped?
If a substance is doped, it means that some small amount of another substance has been introduced into it. More specifically, a substance is doped by a dopant, the latter of which is the minority component. The dopant can be an additive to or a replacement within the bulk material.

In your case, some fraction of $\ce{Ti}$ atoms are replaced by $\ce{Nb}$ atoms (the replacement scenario is the usual one for crystalline materials such as yours). Thus, the physical properties of your substate will be slightly different that those of pure $\ce{TiO2}$.

See this Wikipedia entry for a more complete summary of what I have written here.