As I explained to you in my email this morning, it is a concept that cannot be explained even in a single post. Please refer to what I recommended if you like to learn about it.
After you go through at least some of the Bhikkhu Bodhi’s lectures AND have read some of the posts that I recommended, you can start reading the following section on “Stream Entry”:
“Sōtapanna Stage of Nibbāna”
Of course, if you like, you can start reading that section on the Sotapanna Stage now.
– Since I have no idea about your background, you may well be able to comprehend that material now.
– But if you don’t have that background, it may make more sense after you read some more basic concepts.
In summary, what I mean by ”Stream Entry” is described in the posts at, “Sōtapanna Stage of Nibbāna“