One Becquerel is one nuclear decay per second. The half-life t½ is the time it takes half of the nuclei to decay. Related to t½ is the decay constant: t½ = ln(2)/λ. The significance of the decay constant λ is that it is the fractional rate per second at which nuclei are decaying:
dN/dt = - λ N
So if you multiply λ by the number of nuclei present, N, you'll get the number of decays per second, which is Becquerels.