This pack comes with 4 mask filters, custom shaped to fit AprilMade adult masks using Filti brand nanofiber fabric. 
 
These filters provide a higher protection, and help keep your mask's shape together.  Insert the filti filter into your mask with the green, fuzzier side faces outwards while the white side faces towards you for the most effective filtration.
 
Reuse Guidelines:  This filter cannot be washed traditionally, like the main mask can!  Unfortunately that will affect the filtration. Instead I reccommend a rotation cycle, letting used filters rest for 3 days before reusage.  It can also be sanitized according to the Filti website: ( https://youtu.be/EhiuiJi1uts)  That is why I am selling in packs for ease of use. 
Source: https://www.sages.org/n-95-re-use-instructions/
 
Why Filti?
After researching, Filti is the best material to use as a mask filter.  This specialty fabric is made using nanofiber technology, making it 95% effective at filtering Covid-19 particles.  Studies have shown that using a layer of filti in a 2ply cotton mask is 84-90% effective against Covid particles.
Filti: https://www.filti.com/
Study Sources: 
https://www.maskfaq.com/test-resultshttps://threadsmonthly.com/best-diy-face-mask-filter-materials/#filti