This refers to the yards per game opposing offenses gain against your defense, on average. DEF Rush YPG is Defensive Rushing Yards [allowed] Per Game, and DEF Pass YPG is Defensive Passing Yards [allowed] Per Game. I assume since you've made it through the season you are familiar with the difference between rushing and passing plays already.
In Madden NFL, each team may carry up to 53 players on its active roster during the regular season (as opposed to the 46 you carried on your roster).
Each team in Madden NFL may additionally roster a 12-player developmental Practice Squad (in real life, teams may carry up to 16 players). Whenever your Madden team's season ends - be it by failing to qualify for the playoffs, elimination from the playoffs, or the conclusion of the Super Bowl - every player on your practice squad automatically gets signed to a cheap and short-term contract.
Effectively, you will have contractual dibs for next season on any player you carried on your practice squad at the time your season ended.