The 'metric credits' 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 example below:
The amount of credits you get per month is dependent on your Ahrefs plan:
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 500 metric credits every month. Even if you only use 100, and have 400 left, you will not get 900 the next month. Instead, it will be reset, and you will have 500 metric 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 and 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.)
Don't worry if you are out of credits though. You will still be able to use Keywords Explorer and see the monthly search volume, as well as the cached Keyword Difficulty.
If you run out of credits, you'll either need to wait until they reset automatically or have them reset right away by upgrading to a higher subscription plan.