The 'Harvest,' as mentioned in Mark 4:29, signifies the increase of the church in general and particular.

In Joel 3:13, it signifies the last state of the church before it has been devastated from all good and truth.

'A harvest' symbolizes the state of the church with respect to Divine truth. The reason is that a harvest yields the grain used to make bread, and grain and bread symbolize the church's goodness, which is achieved through truths.

(Referencat: Apocalypse Revealed 645; Joel 3; Mark 4)