Imagine DNA Ligase to be like glue. It brings all the molecules of the newly formed DNA or RNA strands together and permanently binds them with a phosphodiester bond, so that they dont just fall apart. It is a very important enzyme in DNA replication and DNA repair. Do note that it also helps bond Okazaki fragments, which are short strands of DNA formed on the lagging strand during DNA replication, so a continuous strand of DNA is formed.