"a postsynaptic neuron will have an excitatory postsynaptic potential (epsp) when"

When chemically-regulated sodium channels are open and sodium is diffusing into the cell a postsynaptic neuron will have an Excitatory Postsynaptic Potential (EPSP). And if a neurotransmitter binds to a receptor with a reversal potential that is higher than the threshold potential for the postsynaptic neuron, the postsynaptic cell will be more likely to generate an action potential and an excitatory postsynaptic potential will occur (EPSP).