Dog Tags
Top Gun is a move about Navy pilots. Naturally, there are gonna be dog tags.
We're not referring to the actual dog tags you might get at a place like Petsmart, but these necklaces that all military personnel are issued and required to wear. Now, these things show up all the time in Top Gun. Not only can they be seen in scenes where the pilots have their shirts off (the famous volleyball scene, the various locker room scenes), but even at times when the pilots are in their flight gear. Why all the dog tags? Well, for one thing, they make the movie seem more realistic. People in the various branches of the armed forces almost always have their dog tags on, so the fact that we can almost always see the pilots' tags makes us see them as more realistic.
The Last Of Goose
Now, there is one very particular set of dog tags that plays a very important role in the last quarter of the film: Goose's dog tags. They are the only remnant of Goose that Maverick has, other than memories, and they assume an almost religious significance. While waiting on the aircraft carrier before the final fight seen, for example, Maverick looks at them and touches them very gently. We have to presume that he's probably thinking about his friend, and how weird it is to be on the deck of an aircraft carrier without him.
In the second to last scene of the film, Maverick throws Goose's tags out into the ocean. For most of the film, Maverick has had a problem letting go of the past, and Goose's death haunts him for the last quarter of the film. This final gesture from Maverick symbolizes the fact that he has made peace with the past, and has finally accepted Goose's death. He can move forward without the tremendous weight and guilt of that death haunting him.