This question is asking about something that happens during the mitosis phases of the cell cycle. The cell cycle is a cell's life cycle and is typically divided into three parts. Those parts are interphase, mitosis, and cytokinesis.
The cell spends most of its life in interphase. During interphase, a cell will grow, develop, and get itself ready for the cell division that happens during mitosis; therefore, a part of interphase is the replication of the cell's DNA. When that DNA condenses and becomes visible during prophase of mitosis, we are looking at chromosomes. Because the DNA was replicated, each chromosome is attached to a copy of itself. We call these chromosomal copies sister chromatids, and they are joined together at a location called the centromere. Metaphase will have the sister chromatids line up across the cell's equator.
While they are lining up, the centrioles migrate to opposite poles of the cell and send out spindle fibers that attach to the sister chromatids. Those spindle fibers will pull apart the sister chromatids beginning in anaphase, and the sister chromatids will be moved to opposite sides of the cell. Once they have migrated to opposite poles, telophase begins, and the cell membrane will begin to pinch inward. The chromatids (chromosomes) will begin to return to their chromatin status as the nuclear membrane also reforms. The cell will fully separate during cytokinesis, and interphase will begin again.