Spamming means to repeatedly and excessively do something. The word is most familiar today when referring to junk mail, but this usage is predated by spam on BBSs and MUDs, which involved flooding with useless messages.
In Guild Wars, there are several forms of spamming that players might be refering to:
- Flooding the chat window with messages or doodling on the map.
- The practice of repeatedly casting spells that have low energy cost and quick recharge. While not a particularly creative style of playing, it can often be effective when used alongside spells that reward the player for casting, such as Aura of Restoration or Ether Renewal.
- Spamming the enemy with a skill or a general tactic. For example, the tactic of using skills such as Epidemic or Plague Sending to propagate degenerative conditions to the enemy with a single casting is spamming them with conditions.
All of those generally mean to repeat something over and over again.