This is the 10366th most frequent Turkish word.
binmesi
"Binmesi" means "the act of getting on or boarding" in English.
In this sentence, 'binmesi' is used as a nounified verb derived from 'binmek' (to get on/to ride) to indicate the action of riding.
Atın binmesi kolay değil.
Riding a horse is not easy.
Here, 'binmesi' functions in conjunction with 'sırasında' (during) to specify the action occurring during the process of riding a bicycle.
Bisiklete binmesi sırasında dikkatli ol.
Be careful while riding the bicycle.
In this context, 'binmesi' refers to the specific instance of him boarding the bus, acting as a subject of the sentence.