pH is what tells us whether the water is acid or alkaline.
pH below 7 is acidic
pH 7 is neutral <--- middle ground where it is neither basic nor alkaline
pH above 7 is alkaline
Congratulations! That’s all you need to know about pH!
Most fish can adjust themselves to water of different pH as long as it is not too much. If your tap water is around 7 it would be suitable for most fish.
It is better to keep your pH stable than to adjust it all the time because it is stressful (not stressful for you. stressful for the fish!). You need to monintor your aquarium’s pH so that you know when your aquarium pH crashes or shoots up. Most of the time, it is sudden change in pH that kills fish and not because the pH has gone out of the “fish’s pH range”.
One thing to note is that when pH rises, this change can impact on your water parameters. For example, when pH rises harmless ammonia changes to toxic ammonia! (another fish killer).