Heather, open pollinated means that if you save the seeds produced by something you planted and grew, then plant that seed the next year you will get a plant EXACTLY the same as the previous year. Every time, every year, so you only have to buy the seed once to have the same plant every year thereafter. Heirlooms Usually are open pollinated. They are old varieties that have fallen out of popular use but usually have excellent flavor, production, whatever, just gotten scarce over the years. Unless stated on the package of seed most seed you buy in the stores are hybrid which means if you save the seed from the plants you grow, then plant them the next year the likelihood of growing the Exact same plant as previous is not good and the production will not usually be very good either. Most hard core organic gardeners will not use seed unless the seed itself was grown organically, is open pollinated, and not genetically modified in any way.