When the light hits the phototransistor it begins to conduct (the phototransistor is on saturation) and the base of the 2N3904 transistor goes down to almost 0 volts.
The 2N3904 transistor is now in cutoff and no current flows from the colector to the emiter terminals. The number 2 pin of the 555 timer has a high level voltage throuh a 10K resistor (pull-up resistor) and the 555 timer is not triggered.
When there is no light the phototransistor is on cutoff condition (no colector current) the base of the 2N3904 transistor receives enought current through the 2.2K and 10K resistors to saturate the 2N3904 transistor (on saturation condition, VCE is almost 0 volts), then the number 2 pin of the 555 timer has a low level voltage and it is triggered.
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Monostable activation with a phototransistor
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