It means a car, or cars, were probably installed via an online session. If S397 updated the original car after your online session, i.e. base mod, then your downloaded car can't find the old version. Same goes for skins that were designed to hang on, for example, v1.01 instead of the more recent 1.02.
Often when uninstalling a skin pack from a car (More so S397 cars) it also deletes the text from the mft file for that car (The cars ID code) but not the mods files, they are generally still there.
In this case all you need to do is restart the launcher and the sync to Steam workshop will fix it for you.
Generally if S397 do update their car/s to a new version the car will still work (Base Mod) but any third party add-ons like skin packs will no longer work.
For online if a mod is updated the old server code will no longer work if your install is different from the server. Once the server Admin has the update done and reset you'll get the new rfm from the server to allow you to enter it once again.