it's like looking at a ton of gibberish and trying to figure out what it says. it doesn't match the languages, it doesn't appear to match any standards.
I learned a lot from man pages in the 80's and 90's, and if I had to try to learn now from something like this, I'd likely just throw up my hands and choose another career.
My educated guess is that something is trying to italicize various keywords, and whatever text processing is going on isn't working, but producing bogus "In" literal text instead.
It even prettifies those ``'' quotes, notice. This is vanilla older FreeBSD man, using groff, but actually allowing groff to do the things that it is capable of.
As others have noted, the 'In' looks like a rendering bug with whatever terminal emulator you're using. Otherwise it looks pretty normal: `fopen (const char *restrict filename, const char *restrict mode)` looks like a perfectly good C function, and (if you already know C) it's easy to see the parts that don't look like they belong.
I learned a lot from man pages in the 80's and 90's, and if I had to try to learn now from something like this, I'd likely just throw up my hands and choose another career.