The 'SERP updates' are credits used to pull advanced metrics for every keyword you put into Keywords Explorer.
Here's an example:
You can also request advanced metrics for each individual keyword you'd like by clicking on the "Refresh" icon within your keywords reports; see the example below:
These credits reset each month on the day you started your subscription. (The exact date for your limits resetting can be found here).
The unused credits do not roll over to the next month. For example, as part of the Lite Plan, you get 100 credits every month. Even if you only use 20, and have 80 left, you will not get 180 next month. Instead, it will be reset, and you will have 100 credits again.
Will I consume metric credits if I repeatedly analyze the same keyword?
The short answer is: yes.
This is because Keywords Explorer pulls out the freshest data for this keyword each time you request its analysis.
However... If you put two or more keywords into Keywords Explorer and analyze them together, their data will be cached after the first analysis.
That means: Ahrefs will show you the cached data the next time you look at the keyword again, thereby saving your metric credits (unless, of course, you request for the freshest data.)
Please note, you need to have at least one credit left to explore previous cached searches. Once you reach the daily limit, Keyword Explorer will block access to cached reports.