Basically, they are an "attaboy" from a very important person. You see them handed out more to senior people. (Most of us had already maxed out on awards.) If it is your first, put it in your wallet. When you are at the bar and you get "coined", slam it on the table.
Coining - A type of drinking game with coins. Rules are different in different places, but everywhere I have been, it works like this.....
1 person calls out "coin check". Everybody has to pull out their coin. Anyone without a coin buys a round. If everyone has a coin, then they go by the "rank" of the coin. (the rank is determined by the rank on the coin. (Look at your coin, the Admiral's rank will be on it. Only Flag Officers & Sergeants Majors are supposed to have coins.)
I was in the military for nearly 10 years before i got my first one I retired with about 30. How important they are is dependent on where you are at. I did my last tour at the US Sergeants Major Academy & they were harder to get that an award. On the other hand, I once got one from the Assistant Secretary of Defense for shuttling him around Austin, Tx (Pays to have a Dress uniform at the office & after that, I never had to buy drinks - only folks above mine was the Secretary of Defense, the Vice-President & the President). On the third hand, I was coined by several of my students after they were commissioned as Second Lieutenants at Ft. Benning (Exception to the rule above, OCS has class coins). When I was at the National Guard Professional Education Center, we handed them out to the class leaders for each of the courses that we taught.