An outflow boundary occures ahead a thunderstorm complex, e.g., a squall line or super cell. This sometimes happens in a bigger distance to the thunderstorm complex. The reason for this phenomenon are the merging downdrafts from different thunderstorms yielding upward motion at the edge of the downdrafts. The moist and warm air is lifted up when it hits upon the colder outflow boundary air. An outflow boundary can be the onset of new convective cells, even several hours after the highest convection's intensity. If the atmosphere is potential unstable new convective cells can be triggered by the outflow boundary. Because an outflow boundary is a mesoscale coldfront only highly sophisticated NWP-models may cover this phenomenon if optimal input data are available.
More details can be found for example in:
wikipedia