Nesting is the inclusion of blocks within other blocks, which usually means indenting everything consistently. So say the second if (inside the first if) is indented 4 spaces, then the statements within the second if are indented 8 spaces total. Make sense? : ) Here is an example:

    if (this) {
        if (that) {
            do this
        }
        and this
    }
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That second if is nested inside the first if. The indenting just makes things easier to read.