Telomeres are sequences of repetitive DNA at the end of a chromosome. They have been compared to the aglets or protective caps on shoelaces because they protect information from being lost from the ends of chromosomes just as aglets keep shoelaces from fraying.
During cell division, chromosomes are replicated. The enzymes that duplicate DNA cannot continue all the way to the end of a chromosome, so some DNA is lost at each cell division. The presence of telomeres means that the DNA lost does not include information the new cells need to function normally. Following cell division, telomeres are replenished by the enzyme telomerase reverse transcriptase. Telomeres become shorter with each cell division and are eventually destroyed, limiting the number of times a cell can divide.