Pt" means Platinum which is a metal.
"1000" means 1000 ohm at 0 (zero) °C.
The unit "ohm" expresses the electric resistance. Platinum has a positive temperature characteristic (PTC). This means that the initial resistance of the metal increases when its temperature rises.The resistance increases with 3.85 ohm per degree (on the Celsius scale). The characteristic is very linear in the temperature range from -60 to 200°C. Example: Measuring a resistance of 1077 ohm means a temperature of 20°C.