It can mean exactly what it says, that liquid has gotten into your charging port. If so, you should let it dry for quite a bit longer than "a couple of seconds", preferably a couple of hours. Not waiting until it's completely dry can cause more damage.
It could also mean that there is pocket lint or other debris lodged in the port that is causing an incorrect error message. Examine the port using a strong light and a magnifier if you have one. Carefully remove anything you find with a non-conductive tool such as a toothpick.
If you continue to see the message even after letting it dry and cleaning the port, you may have gotten more severe liquid damage. In that case, contact Apple to have your phone evaluated:
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