Unit is the type of measurement you’re working with. Meter for length, kilogram for mass, or second for time. Standard is an internationally agreed upon way of measuring those units. Kilogram is a good example. In Paris there is an object which is internationally agreed to be a standard kilogram. Everything else that represents a kilogram accurately is actually based on a comparison to the standard kilogram. So the standard is the agreed on reference, and the unit is the type of measurement.