If the BMS (Battery Management System) detects high contactor resistance it's likely a sign of impending contactor failure. The car will then send an alert to service and if they get too many of them, I think they schedule a preventative contactor replacement.
The contactors are located in the battery pack and are effectively large high-voltage relays. There are 2 large main ones, one for positive and one for negative and there is also a smaller precharge relay that acts to prevent arcing in the main contactors by allowing them to switch "softly". These 2 contactors act as remote controlled switches that enable the ~400 volt high-voltage power to leave the battery and power things such as the Drive Unit and Climate control system, as well as recharge the 12V battery when it's low.
When you get in your car, you can hear the contactors closing as a "clunk clack" noise under the car in the back, and when the car goes to sleep you hear them open dull "thunk" sound. When the car is off in this state, there is a small 12V Lead-Acid (AGM) battery that powers the so-called "house" or "hotel" loads, such as the Touchscreen, Gateway, Security controller, etc. It also keeps the cellular modem powered so the car can be contacted by Tesla's servers. Because of this, the Gateway keeps an eye on the 12V charge level and when it gets low, it will ask the BMS to close the contactors to enter 12V support mode and recharge it using energy from the main pack. Once it's charged the contactors will again open and the car will go back to sleep. This happens several times each day and night. Since this cycles the 12V battery each time, and it's very small compared to most car batteries, it has a relatively short life. Tesla's 12V battery choice in the early cars was not as good as the red C&D ones they now use, so those rarely even made it to a year old before failure.
