What does the axis of this diagram represent?

This cell is dividing very actively.

He is very sensitive to sound.

He contacted the police to search for the lost wallet.

He tied the luggage with a rope.

It's difficult to collect bamboo tiles in mahjong.

We went on a trip using the highway.

The parkway is one of the best places to enjoy the beauty of nature.

This main road is convenient for commuting.

Please drive carefully not to overlook the road signs.