First a small remark: I'm pretty sure the language you're using is Javascript not Java. They have similar names for stupid historical commercial reasons, but they are completely different languages. It also doesn't help that the code itself looks similar between both languages.
Now about your questions:
how did this come out
As everyone mentioned, i++ is more or less equivalent to i=i+1. arithma's answer is more accurate, but it might confuse you, so for now just focus on the simple equivalence.
why did we use "i is equal to" +i instead of just i
I agree with you this is a stupid quirk of the Js language (a lot of languages share the same issue, and a lot of them don't). Here's the explanation:
Console.log() is a function. This function expects
one argument and will print it
(s string representation) to the console. In other words, you have to give one arg.
"i is equal to" i, in Javascript doesn't mean anything. It's a syntax error. If you want to create a single variable (of type string) you have to use the
concatenate operator.
"a" concatenate "b" == "ab"
In Javascript the concatenate operator is "+". I hope this answers your question and do not hesitate to ask if anything is unclear.