In the circuit shown the GATE connector of the thyristor is connected to the positive terminal of the battery via the push-button switch S1 and resistor R1...
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 Circuit with thyristor and ON switch. (Enlarge)  | 
      
 
 Circuit diagram with thyristor and ON switch.  | 
      
  
If you press on S1, the LED1 lights up. The thyristor has become conducting. Even if you release S1 again, the LED1 remains lit.
With the pushbutton switch S1 closed, a small current flows into the GATE connector. The current accumulates in the thyristor and ensures that the thyristor becomes conducting. The thyristor remains in this state, even if no further current flows into the GATE.
The state of the switched-on thyristor is referred to as ON state
In the next experiment you switch OFF the thyristor...