a fraction is a representation of a proportion, like say you have a pizza and cut it in 6 pieces, each piece is 1/6 of the pizza, .
the total number of available slices in the pizza is 6, you cut it in 6 slices, and a single unit of it or just 1, is 1/6 or one sixth of the pizza.
in short, a rational number is one you can write as a fraction, and an irrational number is one that can't be written as a fraction, a good example of those are π , or e euler's constant or √2.