$_ indicates exactly the last parameter of your last command (GNU Bash)

commandline session

$ 4.2.29 8 507---> ls
Algorithms     bookmark  bugs-general  debian-howtos  firewall	kernel	  linux    README
beautifulwork  books	 config-files  Docs	      https:	language  ovlfose
$ 4.2.29 9 508---> ls $_
ls: cannot access ls: No such file or directory
$ 4.2.29 10 509---> ls vm
ls: cannot access vm: No such file or directory
$ 4.2.29 11 510---> ls $_
ls: cannot access vm: No such file or directory
$ 4.2.29 12 511---> ls Docs
23-awesome-less-known-linuxunix-command-chaining-examples.html	linux-asm.txt  test.html  xargs.html
cairo.pdf							rtl8139d.pdf   wpdb.html
$ 4.2.29 13 512---> pwd
/home/jeffrin/shelter/symmel
$ 4.2.29 14 513---> ls Docs
23-awesome-less-known-linuxunix-command-chaining-examples.html	linux-asm.txt  test.html  xargs.html
cairo.pdf							rtl8139d.pdf   wpdb.html
$ 4.2.29 15 514---> cd $_
$ 4.2.29 16 515---> pwd
/home/jeffrin/shelter/symmel/Docs
$ 4.2.29 17 516--->

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