The answer is that you need to 'rip' the DVDs, which means to make electronic copies that can be stored on your hard drive, NAS, USB stick, tablet and so on. Before we proceed, it should be pointed out that there is some debate as to whether this is strictly legal or not. In some countries it is not, but even if it is then you certainly can't do things such as rip your DVDs and then sell them, give them away, post them online and so on. Rather, the spirit is that you are simply accessing something that you have legally obtained but in a more convenient manner. Obviously we do not condone breaking copyright or anything illegal.
There are many pieces of software around that can rip DVDs. However, they usually cost money. One that doesn't is Handbrake (the rather strange illustration to this article is actually the Handbrake logo) and it is available for both Windows and Mac. It has numerous options and, like all such products, needs a bit of experimentation and fiddling until you find the settings that give the best results. However, if you have an iPad or iPhone then there are built-in presets that will get you running up and quickly. The time taken to rip depends upon the performance of your computer, the length of the DVD, the resolution being used etc but is generally a couple of hours (a lot more in he case of Bluray).
Handbrake works very well, but you'll quickly run into the issue of copy protection. Most commercial DVDs have been nobbled so you can't copy them; in order to do so you need a decrypter program. A free one that many people report success with is DVDFab HD Decrypter, also available for Windows and Mac. This creates a copy of the DVD or Bluray on your hard drive with the protection removed. You then run Handbrake on this copy, which typically generates a MP4 format file which will play on most devices.
If you have a Synology NAS and a Roku streamer