When a mesh is created, the mesh is defined as an array of vertices (points in 3D space). Let is try to imagine a simple square. This is composed of 4 vertices. However, there are two triangles. A triangle is composed of a set of 3 vertices. If we wanted to describe a square by providing 2 triangles, you would then notice that we would have to provide 6 vertices (2 triangles = 2 * triangle = 2 * 3 * vertices = 6 vertices). As the complexity of the mesh increases, so would the number of vertices. An optimization is to provide only the unique vertices and then each triangle would provide 3 indices into the vertex array to say which vertices make up that triangle. Since vertices are commonly reused when you draw meshes, the result is needing to provide fewer total vertices.

So ... to your question ... in order to "remove" a triangle from a mesh, you would get the "triangles" property of the mesh, find the triple corresponding to the triangle to remove and then set the "triangles" property to be the new int array with the triangle to be removed tripple removed from the array.