As far as light has travelled
The observable universe spans as far as light has been able to travel since the Big Bang. Everything beyond, we can never witness.
But somehow, the observable universe stretches more than thrice as far as light has been able to travel since the birth of the universe. The reason behind this madness? The universe is expanding.
The expanding universe
One way to imagine how something that's already endless can become bigger is by imagining a cubic meter: a cube of one meter high, wide, and deep.
Now imagine that the universe is an endless grid of these cubes. Infinite cubes in every direction, neatly stacked on top of each other.
Take a look at your imaginary cube and place a vase with flowers of your choice in the middle.
Now, the cube and everything inside becomes a little bigger—everything expands. Rather than one meter in every direction, the cube is now one meter twenty in every direction. The flowers in their vase increased in size, too, and are still the same size relative to the cube. Everything is the same, yet larger.
Think back to the infinite grid of cubes in every direction. And now envision every single cube expanding like yours just did. That's how forever becomes even bigger.
Like the vase, light is stretched as space expands. Over time, it travels much farther than you'd expect based on the speed of light alone.