Given that attributes are provided as inputs to shaders, in GLSL 300 ES, the attribute qualifier is removed. For example, with WebGL's GLSL 100, you might have the following:
attribute vec3 aVertexNormal;
attribute vec4 aVertexPosition;
In GLSL 300 ES, this would be as follows:
in vec3 aVertexNormal;
in vec4 aVertexPosition;