I believe they both mean “understanding”, I’ve learnt that words in English ending in -ing- use the A + infinitive construct so what is compreensão and why does it also mean “understanding”
Compreender is a verb that means “(to) understand”.
If I may paraphrase what you said: The “present continuous” tense (i.e., talking about something that is still happening) is in English expressed by using the conjugated form of “to be” followed by a gerund (aka a verb ending in -ing), for instance I am swimming, you are running, she is reading). In European Portuguese this tense is expressed by the conjugated verb estar + a + infinitive, for instance, eu estou a nadar, tu estás a correr, ela está a ler.
So eu estou a compreender means “I am understanding”.
compreensão on the other hand is a noun that means “(an) understanding”.
There is a learning note here with more explanations and more examples: https://www.practiceportuguese.com/learning-notes/present-continuous-in-portuguese/?uid=1270
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Nothing to add to @Tomas’s post! 
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