So, a demo is a demonstration of what it is that you can do as an artist. In the past, a demo, or a demo tape, was given to an A&R at a record label so that that A&R, whose role was specifically to find out and recruit talent and develop talent, that A&R could take that to the record label and pitch it to the rest of the team and then sign you to a record deal. Nowadays, there's no need for that, because record labels don't make that much money as they did in the past, so they're not willing to invest as much in an artist just off of sheer talent.
So what they would invest in is somebody who has made a name for themselves. So now the new demo is your numbers: your numbers on social media, your numbers on sites like ReverbNation, sites that can tell the data that conveys to a record label that people like you, people are downloading your music, buying your music. A lot of people use datpif.com, which is a site that you can create a mix tape, which is basically a small demo of your work, and let that be available to people for free.
And in this day and age, by giving away free music, building a following, that is the new demo because the Internet gives you data and gives you numbers to say that this is how much people will buy my music. And if you can show that people are interested in you, then a record label will take you on. And that, in a sense, is the way that, today, you can give in a demo.