Answer:
Rational in the "rational number" sense means "capable of being expressed as a ratio" ... specifically, as a ratio of whole numbers. If you can write it as i/j where I and J are both integers (and J is not zero), then it's a rational number. Any number which can be written exactly as a decimal number that either terminates or repeats (for example, 0.25 (terminates) or 0.3333... (repeats)) is a rational number (those examples are 1/4 and 1/3 respectively).
An irrational number is a number that cannot be written that way. Irrational numbers have infinite, nonrepeating decimal expansions.