whether (something is true)
"Whether" is similar to "if":
I don't know if this is the right one.
I don't know whether this is the right one.
In the example at top, "whether" fits better than "if" because of the sound of the sentence. It would sound a little strange to use "is" and "if" — two short words that begin with "i" — right after each other.
"Whether" is different from "if" because you don't use it in sentences like these:
If you're hungry, just order something from room service.
You're responsible for replacing them if they break.