It’s the diagram that compares physically existing boundaries and the boundaries of what humans can experience. On the left (y axis) you have the time in log scale. The “now” is the begining (left corner down) and the end is the begin of the observable universe. On the bottom (x acsis) you have the distances again in log scale: left the smallest possible right the farthest physically observable (now it’s estimated to be cca 46 billion light years).

The line starting from the lower left ending in the upper right says: the earlier something happened the farther it cam be and remain physically observable (due to the speed of light being finite).

What humans can directly experience is of course small compared to the “all time” and “all space” and it would be invisible without the logaritmic scales used.