When you look in your Nikon D3400 viewfinder to frame a shot, the initial value shown in brackets at the right end of the viewfinder display indicates the number of additional pictures that can fit on your memory card.
But as soon as you press the shutter button halfway, which kicks the autofocus and exposure mechanisms into action, that value changes to instead show you how many pictures can fit in the camera's memory buffer. For example, [r 04] value tells you that four pictures can fit in the buffer.
So what's the buffer? It's a temporary storage tank where the camera stores picture data until it has time to fully record that data onto the camera memory card. This system exists so that you can take a continuous series of pictures without waiting between shots until each image is fully written to the memory card.
When the buffer is full, the camera automatically disables the shutter button until it catches up on its recording work. Chances are, though, that you'll rarely, if ever, encounter this situation; the camera is usually more than capable of keeping up with your shooting rate.