Tank and Spank is the most common type of PvE fight. Tank holds aggro, DPS nuke the mob(s), healers keep the tank alive. The only common complication in fights of this type is aggro - DPS must be careful not to generate more threat than the tank.
The expression is sometimes used derogatorily, meaning that the essential battle tactics for a boss are pitifully simple outside of perhaps a very visible and avoidable AoE. Occasionally this simplicity is used for Gear Check fights, so players can be more certain it is simply their equipment that must be brought to par.