Hybrid flowers are flowers that are a different color than normal. For example, if you were to plant a red tulip next to a white tulip and water them for a while, eventually a pink tulip may sprout up. Hybrids cannot be bought, but ca be sold for large amounts of bells.
Basically, when one flower loves another flower very much...
If you place two flowers next to eachother, whether it's diagonally, vertically, or horizontally, over time if you're lucky enough, it'll produce a "hybrid". Say you place a red and a yellow rose next to eachother, you can get an orange one. It sometimes works with two of the same color, too.
Although two different kinds of flowers, say if you put a white cosmo next to a red rose won't work.
You might even run across a black rose one day and if you have a golden watering can, you can water them and the next day they will be gold roses. I think they sell for about 7,000 bells. Gold roses never wilt, but black roses to make them are tough to get.