Source: Deep Learning on Medium

**What is Convolution?**

In convolution we have almost every time a 3X3 matrix which we multiply with the block of 3X3 pixels. The multiplication is element wise. As we can see in the image below we a 3X3 filter(kernel) which are the numbers below the square blocks and the actual values of the pixels(0–255) which are in the square blocks along with their color. This multiplication is the convolution. After the multiplication we take the sum of all the elements (shown as ‘+’ in the image below). The sum is 51 which is the value of the pixel on the right. This part where we add the values is the convolution layer. The one where the sum is taken as a value and then gives out another value is called the activation layer. In fancy term, this is called RELU( Rectified Linear layer).