是否 is translated into "whether or not; whether; if ", which is a bit misleading. It is probably closest to "whether or not", but it's far from “如果” (if).
是否 ＝ adj 不 adj／verb 不 verb／verb 没 verb
是 means affirmative and 否 means negative in ancient Chinese. The best way to understand 是否 is to know that it has the same grammatical function as the question patterns: adj 不 adj／verb 不 verb／verb 没 verb.
你忙不忙？ ＝ 你是否很忙？ （If the adjective is only one character, we need an adverb here, such as 很，非常，有点儿）
Are you busy or not?
这样说正确不正确？ ＝ 这样说是否正确？ (If the adjective is more than one character, we don't have to add adverbs)
Is it correct to say it in this way or not?
Are you American or not?
你昨天学没学汉语？ ＝ 你昨天是否学汉语了？（了 is required if it's in the past）
Did you learn Chinese yesterday or not?
我不知道他明天会不会来。 ＝ 我不知道他明天是否会来。
I don't know whether or not he will come tomorrow.
不管周末下不下雨，我都要去爬山。 ＝ 不管周末是否下雨，我都要去爬山。
Whether it rains or not in the weekend, I want to go climbing.
"是否" can't refer to assumptions, we have to use "如果" or “要是……（的话）”; vice verse, "如果" or “要是……（的话）” are not able to refer to two opposite situations.
If you can come to the party tomorrow, I would be very happy.
I am not sure whether or not he will come to the party tomorrow.
So that's all there is to know about " 是否". 你们是否明白了？(Do you understand or not?)