Wiktionary
n. The discussion of a sport that happens during the sports off-season.
Wikipedia
Hot stove league is a baseball-related term referring to the sport's off-season. The phrase does not denote an actual league, but instead calls up images of baseball fans gathering around a hot stove during the cold winter months, discussing their favorite baseball teams and players.
The term has also come to refer to the wave of off-season player transactions (contract negotiations, re-signings, trades, free agency, etc.) that occur between seasons. Since most free-agent signings and trades occur during the off-season, this time of significant player transactions (including rumors and speculation about possible trades), is often referred to as the hot stove league or more simply, hot stove.