There are lots of ways of prototyping- from drawing pictures of what something might look like, wireframing or developing the web interface that a user would use, without the data or software integration that might be delivered in a full working service.

It’s important to develop a prototype that’s good enough that the user can provide useful feedback. This is often called a ‘minimum viable product’ - the least you can get away with vs enough to be viable.

Sometimes, startup companies will have just a webpage which ‘pitches’ the product as ‘coming soon’ and offers a box to sign-up for further information. If they get lots of sign-ups, they know it’s an attractive proposition and might develop it further. If there are few, they’ve saved the cost and time involved in actually building the product.

For inspiration

Prototyping for dummies
How to prototype in your web browser
Prototyping through chatbots