Validating your ticket on Munich's public transport network
Most tickets that you can buy need to be validated before your journey. This is done by inserting the end of the ticket into a printing machine which stamps the date, time and location onto the ticket. If you are not sure what is going on, think of it like this:
You buy a ticket. That ticket is valid to make (say) a two zone journey starting at any point that you like and starting on any day that you like. You can keep the ticket in your wallet for a year if you wish and, when you insert it into the stamping machine, you are declaring that your journey starts here and now. An unstamped ticket is not valid for travel and if you are stopped by an inspector, it is no better than having no ticket at all.
Busses and trams have stamping machines on the vehicle and you normally validate the ticket when you board. U-Bahn and central S-Bahn stations have machines outside the entrances to the platforms. You must validate your ticket before you enter the platform. In those stations, ticket inspections are often held not on the trains but at the station exits and it would do you no good to argue that you had not been on a train but instead had just been saying goodbye to a friend.
Outside the central area, the S-Bahn stations have ticket machines on the platforms.