The heart on any listing saves it to the Saved tab of your purchases page. Saving is not buying and never charges you.
Every listing carries a heart. Tapping it saves the item to the Saved tab on your purchases page.
It is not a purchase
Saving costs nothing, charges nothing and tells the creator nothing. It is a shortlist for you.
Where they go
Your purchases page, under Saved. Everything you have saved is there whatever creator it came from, so it works as one list across the whole site.
You need an account
Saving is tied to your account rather than to your browser, which is what lets a saved item still be there on your phone tomorrow. Signed out, the heart is not shown.
If a saved item disappears
A creator can edit, unpublish, sell out of or remove a listing at any point after you saved it, and a saved item is a bookmark rather than a hold. Saving does not reserve stock and does not fix the price — if something is time-limited, saving it will not keep it.