To meld is to completely combine or blend two or more things. If you meld breakfast and lunch, you get brunch. If you meld jeans and leggings, you get jeggings.
Books that are told from the perspectives of more than one character meld different narrators into one coherent story, and complicated recipes meld many different ingredients and flavors. When one thing melds into another, it loses its separate characteristics and blends slowly into the new substance. The origin of meld isn't clear, but some experts guess that it's a combination of melt and weld.