Right, it's a hunk of calcium your snail can use for its shell. Ours are in the shape of a clam shell.
(3 Packages) Weco Wonder Shell Natural Minerals (3 Pack), Small - Total of 9 Shells https://www.amazon.com/dp/B07BMSGZ2B/ref=cm_sw_r_apan_glc_i_5YDBQ92N5RYYE7F2P537
I drop a small Wonder Shell into the tank every 2 - 3 months. It comes in various sizes, so get one appropriate to your tank. I have read that it will dissolve faster if your tank is missing the minerals it contains. In my 5-7g tanks, it dissolves within a week. In my 10g tank (I do use a larger "shell" in there), it will take a month or longer to dissolve.
I generally inspect the snails while they're crawling about for any noticeable shell issues but have never seen any problems - no white areas, no cracks - so I don't feel like I need to really keep dosing the tanks more than a few times a year. For all I know, I probably don't need to supplement calcium (hah), but I do it because I would hate to have the snails suffer from neglect. They help me out, I want to make sure they have a good quality of life.
FYI, Wonder Shell adds minerals to the water, so can have a slight effect on water parameters (I haven't noticed any change in mine). Keep an eye on water quality to be safe.
It's a product called Wonder Shell. It dissolves and releases into the tank, but I'm not quite sure how to control it or how much it releases.
Many people do not know the other part of aquasoils. The buffering capabilities. It's parroted all over that aquasoils is great for plants, which it is, but this important aspect is also missing from the equation.
If you constantly fight against it's buffering capabilities, it will eventually wear out. I don't know how long it takes but I've seen some people say months.
(2HR aquarist site is incredibily good. I'd recommend browsing his blog articles. It's all updated and modern take on planted aquariums)
I've dropped Wonder Shell into the water (link: https://smile.amazon.com/dp/B07BMSGZ2B ) As far as I know, it dissolves in the water and they take it from there. I've maybe seen one of my snails eating on it.
Also, don't let the internet brigade make you beat yourself up over this. You're trying to do what's best by your pets by asking. You could let the water level go down, you could try to get some sort of seal around your filter and anywhere that your lid doesn't cover. Lots of options to consider, just do what works best for your situation.