'Second screens' refers to the way people tend to be using their mobile phone, laptop or tablet while watching TV for example (the second screen being the mobile etc) - a lot of companies try to produce content to get involved in that second screen activity. So for example, X-Factor will have extra content online during the show because they know people will be tweeting about it during.
Obviously same goes a lot for football.
In a stadium it's not a second screen as such because your primary 'screen' is actually watching the game in front of you, but the principle would still be the same.
And to answer the original question, no, I think it would be a horrible idea to have second screen content available at games. We spend too much time on our phones as it is and miss what's going on around them. Encouraging people to sit their with tablets/mobiles out during the game would just kill atmosphere at matches even more. You pay a lot of money to go and watch your team play - watch what is out there in front of you, not the phone in your lap.