Scripting refers to making a script which alters the gameplay of Tribes 2 either on the serverside or clientside. Unlike modding, scripting usually refers to alteration outside of gameplay.
Clientside Scripting
Clientside scripting refers to scripts that only affect the client who uses it. These scripts often make certain tasks easier and "enhance" gameplay. Clientside scripting is extremely controversial, as it can "improve" a player's performance. This is why most tournaments prefer that the players not use clientside scripting (with a few exceptions).
Serverside Scripting
Serverside scripting refers to scripts that operate on the server that affect all players without having them download a client-side addition. Most scripts that don't affect gameplay are serverside, however, some clientside scripts are needed to verify that a player has something. For example, a script written for a reticle pack may want to verify with the server that he actually has the reticles.