A Jupyter Notebook has boxes called cells. It begins with the text input area for code editing, known as the gray box cell, by default. To insert and edit the code do the following:

1. Click inside the gray box.
2. Type your Python code inside it.
3. Click on the play button or press Shift + Enter to run the current cell and move the cursor to the next cell:

Once a cell is run, the relevant data and methods are loaded to the memory and can be used across cells in the same notebook kernel. No reloading is needed unless for an intended change. This saves effort and time in debugging and reloading large datasets.