Pen can refer to several different things:

1. A writing instrument that uses ink. Unlike a pencil that leaves graphite marks you can erase, a pen leaves permanent ink on paper. There are many types: ballpoint pens have a tiny rolling ball that spreads ink smoothly, fountain pens have a nib that draws liquid ink from a reservoir, and felt-tip pens work like tiny paintbrushes. Before modern pens, people wrote with quill pens made from bird feathers dipped in ink. The phrase put pen to paper means to start writing, whether you're drafting a story, signing a contract, or composing a letter.
2. An enclosed area for keeping animals or small children. A pig pen keeps pigs safely contained, and a playpen gives babies a protected space to crawl and play. These pens have fences or walls to prevent animals or small children from wandering off. If you feel trapped or crowded, you might say you feel penned in.
3. As a verb, to pen something means to write it. You might pen a poem, a note, or a speech.