DNA ligase glues together separate pieces of DNA, most usually in the case of the lagging strand when DNA is copied. That strand is copied one small piece after another as the double helix is opened bit by bit. Ligase glues those pieces together by making the covalent bonds of the phosphate backbone between successive pieces of the lagging strand.
Work Step by Step
To get this answer, look at ligase--its name means that it is an enzyme that ligates or ties together. Then consider where it is mentioned in the text with regard to DNA copying and the lagging strand.