It definitely does not mean that you didn't understand that topic at the time. Don't worry, I'm sure you understood those topics entirely, but understanding something and memorising it are not always one and the same.
There are two parts to memorising:
1. Creating mental associations and connections
Without both of these you can understand something perfectly at a given moment but forget it over time (which I'm sure is what happened with you).
I've given a little more detail below but the short answer is above :)
Mental Associations: Memory works by creating associations and connections in your brain - this is the foundation and the key. It's why one of the most ancient memory techniques in the Loci Method, where you create associations between the random objects you want to memorise (or numbers, or whatever they may be) and specific places either on a journey you know well (such as the route to work) or a location you know well (such as a house). Things that happen to you often have inbuilt associations to your life, no matter how minor those events were and for some people it's easier to create those connections than others.
This is also why the other answer here refers to making diagrams to explain topics - one of the popular ways to do this is through mind mapping (a term coined by Tony Buzan). In fact, when learning how to speed read, students are often taught this method for organising the information they are capturing to aid the creation of those mental associations.
In your case, if you're studying a topic - and especially if it's particularly detailed or technical - you may simply not be actively creating these long lasting connections in your brain, but rather be studying that topic to understand it in the short term and have a good grasp of the subject without committing it to long term memory. This is perfectly possible as we often have to make a conscious decision to do this for topics which do not inherently form connections for us.
Revision: But even if we create lots of connections and this helps us remember something for a little longer, it is very unlikely you will remember it for a good amount of time without revision. In order to retain information you have studied, you need to revisit it regularly, otherwise it is both natural and healthy for your brain to forget.
Once you have your notes in a condensed format (my preference for mind mapping comes from being able to see a lot of information in one place quickly), by spending just a few minutes reviewing them regularly you can remember a very large proportion more than if you don't look at those notes again. The method I was taught was that the optimal times to review your notes is after:
If you really want to remember a topic, I would suggest jotting down in note form what you can remember from the last time you revised your notes before you even look at your original notes. This will also exercise your ability to recall.
Lastly, I know you mention that you can't use paper (which is what I use mostly for mind mapping) but there are many mind mapping applications out there (some free, some paid for). In fact, I'm going to go and ask that question now on the Webapps stack exchange, I'd quite like to try some out!
Hope this helps :)