To dehydrate ethanol one uses concentrated sulfuric acid:
$$\ce{C2H5OH ->[\text{conc.} H2SO4] C2H4 + H2O}$$
but to go in the reverse direction, dilute sulfuric acid is used:
$$\ce{C2H4 + H2O ->[\text{dil.} H2SO4] C2H5OH}$$
Why is one concentrated, and one dilute? For that matter, how does the sulfuric acid even help in the first place?