Okay kiddo, so imagine that you have a toy box and you want to put all your toys in it. Let's say you have 10 toys, but your toy box can only hold 5 toys at a time.
Now, instead of putting all your toys in the toy box right away, you decide to keep some of them out and only put 5 toys in the toy box. Then, when you want to play with a different toy, you take one of the toys from the toy box and put it back in your pile of toys.
This is kinda like what allocate-on-flush does. When you're using a computer and opening different applications or programs, the computer sometimes has to use something called "memory" to help those applications run smoothly.
Allocating memory means setting aside a certain amount of space for the computer to use for a specific task. So, if you're opening up a really big file, the computer might need to allocate a lot of memory to help it process the information in the file.
Allocate-on-flush is a way of managing that memory. Instead of allocating memory all at once, the computer only sets aside a little bit of space for each task and then "flushes" it, or clears it out, when it's done.
So, it's kinda like putting your toys in the toy box one at a time and only keeping a few toys out at any given moment. This makes it easier for the computer to handle lots of different tasks at once without running out of memory or slowing down too much.