Plant macrofossils are basically any part of a plant that is preserved in our peat cores. For example, when we look at the peat through a microscope, we can see the leaves and stems of different mosses, lots of different roots and the leaves and stems of other plants such as heather or various grasses. We are interested in plant macrofossils because looking at how they change over time (remember that the deeper we go in our cores, the older things become) can give us information about the growing conditions on the bog surface, which are linked to climate.
For example, we know that certain sedges or rushes, or particular species of moss, prefer to grow in much wetter conditions. Others prefer to grow in much drier conditions. We know this by studying how and where the same species grow today. So if we take regular samples down through our core and see that the plant communities are changing from those that prefer wet conditions to those that prefer dry conditions, we can infer that the climate at those times was wetter and drier too.