What does this mean?

I have seen numbers like 10{40}10, F49, and H5, I know what those mean, but what I don't know, is this: {A, B, C, D}. Example: {10, 8, 2}. I have no idea what these mean.

{10,8,2} = 10{8}2
{A,B,C,D} = I looked into it for 6 hours and still don't understand it

Those numbers are called BEAF numbers

You should know that 10{8}2 = 10^^^^^^^^2

We can do something called expansion, it looks like this 10{{2}}8
(notice the two {}s)

10{{2}}8 = 10{10{{1}}8}8 = 10{10{10{x}8}8}8 (note: that x is just a number, it wont really affect the number because its so massive as long as its not too small)
You can expand this to have three {}s
10{{{1}}}8 = 10{{10{{x}}8}}8 which is absolutely massive, its just 10{{10{{x}}8}}8

Another way to write 10{{2}}8 is {10,2,8,x}, replace x with the "just a number" i talked about earlier
you can extend this as many times as you want, like {10,10,10,10,10,10}

theres also stuff like 10&6 and {10,10(0,1)2} but im not gonna talk about those

im not very good at explaining things

Yeah, Bowers' arrays are only really intuitively understandable up to 4 entries.