A mating cycle is one insertion and one removal. The rating on a datasheet is the number of cycles the connector was tested to under stated conditions, after which its contact resistance must still be within a stated limit. Some connectors are rated for only a few tens of cycles, others for thousands.
The number depends mostly on the contact design and plating. Gold plating generally tolerates many more cycles than tin, and thicker plating lasts longer. The test criteria matter as well, so two datasheets with the same number are not always directly comparable.
The rating applies to a clean, properly aligned connector. Misalignment, dirt, vibration, and use with a mating part of a different plating can shorten life. If your product will be plugged and unplugged in service, choose a connector rated well above the expected number of cycles, and treat the rating as a design target and not a guarantee.
