A dicot is a flowering plant with two embryonic seed leaves, rather than the single embryonic seed leaf associated with monocots. Embryonic seed leaves, also known as cotyledons, are an important part of plant development, and monocots and dicots develop in different ways right from the very start. This type of plant may also be referred to as magnoliopsids, referencing the class to which they belong.
The term is actually a shortening of dicotyledon, the proper term for plants in this class. Since this word and “monocotyledon” are a bit of a mouthful, most biologists shorten them. Both belong to the larger phylum of flowering plants, sometimes known as angiosperms, with thousands of individual species all over the world.
The cotyledons aren't the only difference between monocots and dicots. Monocots have energy reserves in their cotyledons that are used to help the plant grow and develop, a key difference in germination. The two have different pollen structures, with dicots classically having three grooves in their pollen, rather than one. The flowers have petals in multiples of four and five, and their leaves have netted veins. This means that even after a plant matures, making it hard to know how many cotyledons it had, gardeners can still determine which type it is.
Many crops are dicots, including plums, beans, and peas. Plums are famous for their five-petaled flowers, which betray their dicot status, and many people have raised peas or beans in the classroom or garden, noting that these plants sprout with two tiny cotyledons. Edible dicots have seeds with two distinct halves, referencing the potential to develop two cotyledons. Other crops, like wheat, coconuts, and rice, are monocots. Many grasses, in fact, are monocots.
While the distinctions between monocots and dicots might seem petty, these differences help to classify the flowering plants, breaking them down into more manageable groups. Taxonomy is all about finding traits in common, looking for things that can be used to group some things together while excluding others. The monocot/dicot split allows biologists to look at two main categories of plants within the angiosperms, breaking these categories down into even smaller groups further on down the line until you arrive at individual species.