Open-pollinated
A variety that reproduces itself: saved seed grows true to type, provided the flowers weren't crossed with another variety. Outdoors, that's never guaranteed.
An open-pollinated variety reproduces itself. Save seed from it, take reasonable care, and the next generation looks like the last. Seed catalogues shorten it to OP.
The word "open" causes most of the confusion. It describes how the variety is maintained, not a promise about your particular pod. Open pollination is still pollination in the open, where an insect can arrive carrying pollen from the chinense three pots along. An OP variety grown among other chillies produces crossed seed as easily as anything else does.
Against F1
An F1 hybrid is remade each season from two parent lines, and its saved seed scatters into variable F2 plants. An open-pollinated variety is stable in itself, so a grower can keep it going indefinitely without going back to a seed company. That is the real distinction, and it is why seed-saving networks and landrace projects deal almost entirely in OP varieties.
Saving it properly
Select plants that are typical of the variety rather than the single most impressive one, take seed from fully ripe pods, and isolate the flowers if purity matters to you. Anything that comes up off-type next season should be pulled rather than bred from.
An heirloom is an old open-pollinated variety with a history attached. Every heirloom is open-pollinated; plenty of open-pollinated varieties are recent introductions and not heirlooms at all.