What are the weird things in quotation marks when formatting a string?

Sometimes, when I look at other people's scripts, I usually see something like string.format("%d:%d", something, something), and I am really confused on what those in quotation marks represent. I don't know what they're called, and I tried looking them up to see what they are but I haven't found a single website that explained even the slightest about what they are.

Can anybody in the comments explain to me what they do? And if there is an article, please give a link

Thanks in advance

print(string.format("%s,%s,%s", 1, 2, 3))

Output --> 1,2,3

%s is a tag that is replaced with the following argument