Check if variableName is True or not

How does it work ? What does the if condition do ?

function variableName(name) {
    for (var i = 0; i < name.length; i++) {
        if (!('a' <= name[i] && name[i] <= 'z' ||
            'A' <= name[i] && name[i] <= 'Z' ||
            '1' <= name[i] && name[i] <= '9' ||
            name[i] === '_')) {
            return false;
        }
    }
    if ('0' <= name[0] && name[0] <= '9') {
        return false;
    }

    return true;
}

I understand that he check the validation of the variable name , but I didn't understand what 'a' 'z' || 'A' 'Z' || refere to ?!