Each "-illion" is 1000x bigger than the previous one.

If you have some cubes 1 cm on a side (about the size of a sugar cube), you can make a bigger cube out of them with 10 little cubes along each edge. Now you have 1 big cube made from 1000 smaller cubes.

Your bigger cube is now 10cm x 10cm x 10 cm. Easy enough to pick up in one hand.

Now do it again. Make a bigger cube with 10 of those cubes along each edge. Now you have a cube 1 meter on each side. Too big to pick up by hand but it would still fit in the back of a pickup truck.

This 1-meter cube contains 1 million sugar cubes.

Do it again: With 1000 of the previous cubes, make a cube 10 meters on a side. This cube is the size of a 3-story house, and it contains 1 billion sugar cubes.

Now do it once more: With the house-sized cubes, make a 10x10x10. Now the cube is about the size of a football stadium. It contains 1 trillion sugar cubes.

Take 4 of these stadiums, call each sugar cube $1, and you have the market cap of Nvidia.

[Note: This is US usage. In older UK English, some of the "-illion" words mean different things than they do here.]