Basic meaning: having the most plants in a given space growing / cropping for as long as possible through the year.
There are a few different ways to do this including:
2) Having the same plant in succession: once the first crop is done, planting the same crop in the same place right after. i.e. lettuce planted once the first lettuce has been picked.
3) 2 or more crops / plants planted together (see also intercropping / companion plants): I.e. the three sister technique used by Native Americans where corn, beans and squash are all grown together for the support, nutrients, and other benefits each crop provides the other.
4) Same plant, but with different maturity times. i.e. planting summer and autumn raspberries together to get a longer cropping period.