The radix point is a generalised form of the decimal point, as the decimal point is specific to our normal base 10. The term is most often encountered when dealing with unusual bases.
The radix point seperates the quotient (on the left), from the mantissa (on the right). For more details of their significance, see Base.
In American and British notation, it is represented by a period or full stop but European mathematicians prefer a comma. In English it is read "point", eg 2.3 is "two point three" - but in France that would be "deux virgule trois"; literally "two comma three", and written 2,3