like 5V pin is vcc/supply pin and Gnd pin is grounding so what is the significance of 3v3 pin?
You can use 3v3 pin as well, for using the LDR with a voltage divider circuit.
The 3v3 pin (5v pin as well) in bolt has a high current draw, therefore it is suggested not to use it without any resistance.
There are components and boards which require 3.3v power supply. In those case, this pin can be use used directly to power them as well.
got it thank you for the reply
The 3v3 pin is a 3.3V volt power supply pin. Some modules and sensors work with lower voltages. When connected to a higher voltage supply, the modules/sensor chips will burn out due to over-voltage. On the other hand, certain modules require higher voltages to operate. For this purpose, the Bolt module contains two voltage supplies. The 3.3V supply works for modules that require lower voltages while the 5V works for modules that require higher voltages. There are modules that require much less voltage. This can be achieved by dividing the voltage using proper resistors. Since most modules operate on these two voltages - 3.3V and 5V, these two are provided on the board (Arduino, Bolt, Raspberry Pi).
Lower voltage means lower power consumption.3v consumes 50% less power than 5v.So 5V draws high current hence it is recommended 3.3v