It's a measure of battery health. The more cycles a battery has been through, the less capacity it has to store power in future recharges. So the more cycles a battery has been through, the less time it will be able to keep your laptop running when starting from a full charge.
A cycle is when a battery has fully discharged it's maximum power storage capacity. e.g. if a battery is fully charged and then run down to zero, that equals 1 cycle. If a fully charged battery is run down to 50%, recharged to full, and then run down to 50% a second time, that also equals 1 cycle.