Okay, imagine you have a shape that's made up of four triangles (which are like three-sided shapes). This shape is called a tetrahedron.
Now, a Heronian tetrahedron is a special type of tetrahedron where all of the sides have whole numbers for their length. So, for example, if the sides of the triangles are 3, 4, and 5, then the tetrahedron they make would be a Heronian tetrahedron because all of those numbers are whole numbers.
But that's not all! A Heronian tetrahedron also has a special property where the area of all four of its faces (the triangles) are also whole numbers. So, if the triangles in the tetrahedron have an area of 6, 8, and 10 square units, then the fourth face would also have an area that's a whole number.
So basically, a Heronian tetrahedron is a special type of shape made up of four triangles where all of the sides and face areas are whole numbers!