Aggressiveness (or hostility) refers to the behavior in which some monsters will attack players automatically if they come within a certain range. This heeds players more in RuneScape Classic than in RuneScape 2, due to the combat system in RuneScape Classic preventing players from retreating for the first 3 rounds of combat. Aggressive monsters will also usually follow a retreating player for a certain distance.
However, with most aggressive monsters, if a player reaches a combat level that is one more than twice the monster's level, it will not be aggressive to that player anymore.
Also, in RuneScape Classic rather than RuneScape 2 the monsters that are aggressive will never stop, so training in RuneScape Classic is a little bit more afkable which is good. Since RuneScape Classic is harder to gain combat stats and as long as you have good Defense you should not die.