How does tactical damage rating effect damage?
One point roughly corresponds to 1% extra damage. Just hoover over a skill and you can see how much the damage increases when you increase your damage rating.
Tactical Damage Rating (TDR) is found on all Minstrel, Runekeeper, and Loremaster mainhand legendary weapons. The basic gist is, without an LI in your hand, your tactical damage skills act as if you were level 50*. With an LI, your skills act as if you are level 50 + the LI's TDR. In essence, TDR is your effective skill level for damaging skills. (It's likely that the Tactical Healing Rating on your book works the same way for healing skills.)
You can review the sticky for more gory details. Note that "Tactical Offense Rating" was removed with the Isengard expansion and was replaced by "Tactical Mastery", and I have no idea how the math presented in the Loremaster post may have changed as a result.
[*] Unless you're lower than level 50, in which case your tactical damage skills act at your actual level. The salient point is that your tactical skills stop levelling at 50, and your LI's TDR is the only way to raise them higher.
TDR is basically the DPS value of your spells as a Will-based class. The higher the TDR, the more effective all the bonuses you get from traits, items and tactical mastery increase your damage output. Note that TDR does not increase healing.
Heal =(Base + Rating)* Outgoing Healing * Incoming Healing * [Crit Muliplier]
Above is the equation for working out your heal amount (its the same for damage). the TDR and THR come into the rating component. You can see it increases the base amount of a skill. The increase depends on the skill involved. It is an important value because it interact multiplicatively with outgoing healing.