If you have a single solar panel connected to your YC500A microinverter (if there are an odd number of modules in a string, for example), you need these protective caps to cover the unused connectors on the unused side of the inverter.
From the manufacturer:
[T]hese caps should be used to protect the unused MC4 wires on the YC500 A-NA. Female cap is used to cover and protect unused MC-4 male connector coming from second pair of YC500 A-NA DC connections. Male plug is used to protect unused MC-4 female connector in same application.