Rational numbers represent exact quantities. On the other hand, a floating point number is an approximate value. While rationals can grow to any required precision, floating point numbers have limited precision, and manipulating them is usually faster than manipulating ratios or bignums.
Introducing a floating point number in a computation forces the result to be expressed in floating point.
5/4 1/2 + . 1+3/4
5/4 0.5 + . 1.75
Floating point literal syntax is documented in Float syntax.
Integers and rationals can be converted to floats: