Hi,
Welcome to the forums.
There are multiple ways you can use to watch movies from your iPhone and/or iPad.
1. Buy/rent from iTunes. If you have an iPhone or iPad, you likely have an iTunes account as well. If you set up your account with payment information, you can buy/rent movies using the iTunes app on your iPhone/iPad with a simple tap.
2. Digital Copy movies. Many DVDs and Blurays come with a "Free Digital Copy" code, which you can enter on iTunes. Similar as purchasing/.renting the movie from iTunes, the code will give you access to the movie as well, but you don't have to pay for it sincew you paid for the Bluray/DVD that it came with.
3. If you want to watch movies you already purchased on DVD on your iPad or iPhone, and they didn't come with a Digital Copy code (or it expired) you need to convert them to a video file you can import into iTunes.
There are plenty of tools that can do this, but one of the easiest and cheapest tools for the job is DVD Catalyst.
For a detailed, easy to follow guide on how to convert your DVDs to play on your iPad or iPhone, have a look here:
https://www.tools4movies.com/dvd-catalyst-apple-ipad-4-guide-2/