The meaning of "a" in an awk command?

The meaning of "a" in an awk command?

I have an awk command in a script I am trying to make work, and I don't
understand the meaning of 'a':
awk 'FNR==NR{ a[$1]=$0;next } ($2 in a)' FILELIST.TXT FILEIN.* > FILEOUT.*
I'm quite new to using command line, so I'm just trying to figure things
out, thanks.