Rational numbers are numbers that can be expressed as a fraction (ratio). Irrational numbers can not be expressed like that (like sqrt(2)).
To prove your statement, assume the opposite until you have a contradiction.
If the result of adding them would be rational, then your irrational number can be expressed as the difference of two rational numbers, which itself is also a rational number. That cannot be, because it should be an irrational number. This contradiction forces that rational + irrational = irrational.
You can reason the same way for multiplication. Suppose rational * irrational = rational, you find that your irrational can be expressed as the fration of two rationals, which is a contradiction.