A Bronsted-Lowry acid is any substance which is a proton donor. A Bronsted-Lowry Base is any substance which is a proton acceptor. For example
HCl(aq) + H2O(l) -> H3O+(aq) + Cl-
In this example, the acid is the HCl molecule, because it donates the proton (i.e. the H atom) to the water molecule, which is thus the Bronsted-Lowry base as it accepts the proton.