Tagalog is one of the official languages of the Philippines. The other is English. This means there’s a lot of fun crossover between the two. English words get Tagalog-ized and Tagalog words get English-ized. One example is above.
In Tagalog it’s easy to change a noun into a verb, a verb into a noun, and an adjective into whatever you want. All you have to do is change the prefix. This Tagalog convention is often applied to English words. Here, the adjective “checkout” (as in “checkout stand” or “checkout counter”) has been transmogrified into a noun. And we all know that when a noun refers to more than one of the same object it needs to be plural.
So next time you’re in the Philippines avoid confusion and tell your friends that when you’re done shopping you’ll meet them at the “checkouts.”