Okay, so you know how there are different ways people talk depending on where they come from, like how someone from Texas might say "y'all" instead of "you all"? Well, sometimes different languages also have different ways of talking within the same language group.
In Turkey, there used to be a group of people who spoke Greek because they had Greek ancestors. But over time, they also integrated with the wider Turkish community, and their way of speaking Greek changed to reflect that.
This is what people mean when they talk about the Istanbul Greek dialect. It's a way of speaking Greek that is unique to Istanbul because it has been influenced by the Turkish language and the Turkish way of speaking.
For example, some words might be pronounced differently than in standard Greek because they have been adapted to fit better with the Turkish language's phonetics. Or there might be new words or phrases that have been borrowed from Turkish and incorporated into the dialect.
So essentially, the Istanbul Greek dialect is a blend of two different languages and cultures that have come together in a unique way.