This is the 16694th most frequent Turkish word.
yüklenirken
"Yüklenirken" means "while loading" in English.
In this sentence, 'yüklenirken' indicates an ongoing process, namely the loading of the website.
Web sitesi yüklenirken biraz beklemelisin.
You should wait a bit while the website is loading.
Here, 'yüklenirken' is used to describe the time during which the loading of the program was happening.
Program yüklenirken hata oluştu.
An error occurred while the program was loading.
In this usage, 'yüklenirken' conveys the context of a file being uploaded and the ongoing time frame of the action.