Can someone please explain to me how President Obama "opened our borders"?

I was having a discussion on Facebook in the middle of which I said that I was more or less satisfied with Obama's presidency. Then another guy I knew joined the conversation saying that Obama was a failure because he "opened our borders", he made a "deal with the devil" (Iran), and he gave us another "monstrosity of a social program that was designed to fail". I asked how Obama opened our borders, when did that happen? And the original poster asked if I had been asleep all eight years of the Obama administration. I said I knew he didn't talk about building a wall, but what policies did he implement that "opened our borders"? And he replied that it was not his job to educate me, do the research myself. I did a quick Google search and all that came up was stories about how Obama's DHS secretary recently said that Democratic presidential candidates' proposals to decriminalize border crossing is tantamount to open borders. So then I typed up this question here on SW, and that's all I feel compelled to do to get to the bottom of this. So what did I miss? When and how did Obama roll out the welcome mat to illegal immigrants in between all the deportations he oversaw?