Field blends are where different grape varieties from the same field, are harvested then fermented at the same time. This means everything is single-vineyard.
Typically, grapes are harvested and fermented at separate times as some grapes take longer to ripen than others. A field blend, therefore, will produce an interesting wine as different characteristics of grapes are picked up.
An extra layer can be where the grapes are fermented together, which is called co-fermentation, not just at the same time.
This all adds up to some interesting wines and what I’d consider a pretty strong expression of terroirs over anything else. It makes for particularly interesting wines in vineyards where there’s multiple varieties that all have different ripening cycles.