It means the server is using an old version, older than the version you are using.
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It means the server hasnt updated to a newer version of the game.
An outdated server is a server that is not up-to-date with the version you're playing on. For example, if you are on 1.8 and the server is on 1.7.9, it is considered outdated
Client outdated means that the client (the software used to connect to the server) is out of date. You need to update your version of Minecraft.
it wont you need to download the server thing of the minecraft website and replace the old one with it.
Either you don't have the latest minecraft client or the person is using the program session stealer which when someone joins the server it kicks them with the message outdated cleint and takes their il logs onto a specified server your oped the does /op command to gain admin