There a couple explanations on the previous page, but in a nutshell:
There's a summer time window to sign North American trained players who went undrafted but have remaining draft eligibility. Window closes around the time when the player has to return to their junior teams.
The most notable guy to go this route might be Martin Jones who went undrafted in 2008 but impressed at Kings prospect camp and was signed to a deal. Michael Sgarbossa went undrafted in 2010, earned a deal with the Sharks a few months later, and won the OHL scoring title in 2012. If I recall correctly, the Sharks signed a few guys like this (J.P Anderson was another one) back when they had used a lot of draft picks in deadline deals.
Teams have to deal with contract limits, so they do have to be judicious with who they sign. A guy like Nathan Noel went undrafted in 2015 and attended Anaheim's prospect camp but wasn't extended a contract offer. Noel then got drafted by Chicago in 2016.
I've heard/read stories about how teams typically will have a handful of guys in their top 100 go undrafted. I sat through the entire 2011 Draft and after the last pick was made, I saw several guys from different draft tables dash into the crowd to talk to presumably undrafted prospects and maybe invite them to camp.