Is sodium bicarbonate acidic or basic?

It's both a weak base and an (even weaker) acid.

base (proton acceptor) HCO3− + H2O <--> H2CO3 + OH− with pKb = 6.3

acid (proton donor) HCO3− + H2O <--> CO32− + H3O+ with pKa = 10.3

It is used to neutralize acids in the stomach as an antacid.