Why is NaHCO3 a basic salt?
The thing is that $\ce{HCO3-}$ can act as an acid and a base. These substances are  called amphiprotic (other examples include $\ce{HSO4-}$ and $\ce{H2PO4-}$). So it can donate a proton to water to form $\ce{H3O+}$ but it can also accept a proton from water to form $\ce{OH-}$. So really, the  question is which process occurs to a greater extent?.

You can't answer that question by simply looking at the molecule. Instead we need to know the $\mathrm{K_a}$ values for $\ce{H2CO3-}$ and $\ce{HCO3-}$. By comparing these values and doing a few calculations we can determine whether $\ce{HCO3-}$ is basic or acidic. It turns out $\ce{HCO3-}$ is actually basic.

However I am assume that equilibrium constants are probably above the level that you are learning at. So this is just one of these things which you should just know. For example you should know that despite $\ce{HCO3-}$ being basic, $\ce{HSO4-}$ and $\ce{H2PO4-}$ are actually acidic.