If substitution of a nucleophile has happened, does that mean it is an E or an SN reaction?
First order just means that the reaction depends on the concentration of only one reactant. In other words, one species is participating, like in a half-life reaction, a unimolecular reaction.
Second order means that the reaction depends on two of these "first-order" reactants---a bimolecular reaction, or one "second-order" reactant.
You can see how
Order corresponds to how:
is first order in
is second order in