blockhead is incorrect. there is no catch-all method of giving someone eval trust.
eval is essentially console access. anyone who has eval can potentially delete all your blockland files, or upload files to your computer (but can only "run" them if they are blockland files). it is normally only accessible on the computer hosting the server.
eval mods/addons allow users to use eval ingame, and generally come with other useful features. port's eval, for example, allows players to give other players eval access, supports multiple-line evals, checks for variables like %player and %client and automatically replaces it with the correct variables, and dumps console output to chat.
generally if you aren't knowledgeable enough to setup eval on your server without help (or with only minimal help), you should not install an eval mod. there's too much potential for a malicious or inexperienced user to either crash or screw up your server.
