A landrace is a food crop with lots of genetic diversity which tends to produce stable yields under marginal growing conditions. Landrace crops are adaptively selected for resilience in tough conditions. The arrival of new pests, new diseases, or changes in cultural practices or in the environment may harm some individuals in a landrace population, but with so much diversity many plants are likely to do well under the changing conditions.
In the case of mostly self-pollinating plants like peppers, tomatoes, beans, wheat, lettuce, and peas a landrace may be thought of as many distinct varieties growing side by side. In the case of out-crossing plants like melons, squash, brassicas, spinach, or corn, a landrace may be thought of as an open pollinated population with tremendous genetic diversity. Many of the seeds in an out-crossing landrace end up being unique F1 hybrids.
Commercial seeds tend to be highly inbred and in order to produce maximum yield require regularly scheduled weeding, and inputs of pesticides, herbicides, and fertilizers. Small scale farmer's and home gardeners are generally better off planting landrace seeds that were grown nearby, because then the plants are better adapted to local conditions. The most resilient practice is to grow one's own seed. Hybrids are often incorporated into landraces which increases the diversity of the genepool.
Open Pollinated or Heirloom vs Landrace
Seeds are sometimes described as being 'heirloom' or 'open pollinated'. These varieties can be highly inbred, often having originated from a single seed. Heirloom and open pollinated varieties may also be genetically diverse, but they are seldom as diverse as a landrace.
You may grow your own landrace seeds by planting the seeds from 3 to 10 varieties of a species and saving the seeds from the plants that grow best for you, or you may get them from a supplier of landrace seeds.