Ligase is an enzyme that joins strands of DNA together.

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Ligase is an enzyme (we know this instantly because it ends in -ase), that ligates or "joins together" strands of DNA.

During DNA replication, there is a leading and a lagging strand, due to DNA polymerase (another enzyme) being only able to travel in one direction, 5' to 3', or Five prime to three prime (these names come from the position of the Carbons on the DNA base molecule), and because DNA strands are antiparallel. and as a result there are breaks in the strand of DNA. Ligase joins together these breaks in the DNA, to fix the strands into one continuous strand.