The meaining is the lorry is sealed by customs in the country of departure and can cross other countries still sealed without needing to be opened by customs in the intermediate countries. e.g. a truck of goods from Italy destined for the UK crossed the Swiss & French borders but isn't examined and pays no duties until it arrives in the UK where UK customs examine and levy any taxes.
In practice, it is hardly used in Europe anymore, due to the opening of all the borders in the EU. Although technically Switzerland is outside th EU, you don't have to have a TIR Carnet (The TIR paperwork) as long as you have your T1 & T2 forms (the same sort of paperwork but much simpler). The only trucks that still use TIR usually come from further afield than Turkey (ie Asia and Asia Minor).