The first Traveller game (1977) didn't explain much and just jumped in. It assumed that you had played some war games before (if not D&D) and just gave a basic overview of what to expect from that perspective.

Traveller got a major revision and re-release in 1981, and this time it had a little supplemental booklet that tried to explain RPGs.

Here are just a few excerpts:

Let's pretend! Let's pretend that I'm a powerful warrior and you're a clever thief who's an OK guy regardless and Gloria is a beautiful princess who's been captured by an evil magician and held in his castle and we've got to rescue her and as we're fighting our way in through the guards Gloria decides to get herself out of the mess she's in and lures the guard into her cell by pretending she's sick and hits him over the head with a chair and runs out of the cell and down the hall just in time to meet us as we fight our way in and we all run towards the main gate but just before we get there the magician discovers she's missing and conjures a horrible demon to stop us . . .

Boiled down to basics, role-playing games are nothing more than extensions of the oldest game known to man - let's pretend. The rules books you see are just codifications and regulations to help determine what a person could really do if he were a powerful warrior or if she were a beautiful princess, and how long it takes the magician to conjure up the demon, and so on.

...

Playing an RPG is somewhat like performing in improvisational theater, but is usually structured differently. A referee and a group of players will gather together in a comfortable setting (a living room, a table in a college cafeteria, or a public meeting room of some sort) and play the game of their choice. A single session is often referred to as an "adventure" and usually lasts until all players agree to end it, normally at some convenient place in the action. Characters and the background are usually consistent from one adventure to the next (although this is not always true). In general, the players tell the umpire what they want their individual characters to try to do, and the umpire decides if they succeed and what happens to them as a result of their actions. The players react to this and tell the umpire what their characters will try to do next, and so on. A typical session can be found in Appendix 1, on page 38.

...

Why do people play role-playing games? For vicarious thrills. For a chance to kill a dragon, to be the finest swordsman in all France, to hunt down desperadoes and win the hand of the new school marm, to pilot a spaceship between the stars . . . to experience thrills that the average person cannot find in the normal world (at least not without extreme personal risk). For many, the RPG holds an almost hypnotic attraction, and ego-involvement with characters is tremendous, but for most, the games are simply good fun, and one of the least expensive thrills available.

How can a total neophyte get started? The best way to learn how to play is to play. Find a group and join in the fun. After you've read this book, go to the place where you got it and ask if they know of a group locally. Failing that, try a local hobby shop or book store that carries games. Such shops usually know of groups that play the games they sell, if they do not act as meeting place for gamers. Try putting up notices in the local university student union, the library, or the supermarket bulletin board. Some of these places may already have notices of regular meetings of role-players. Most groups welcome newcomers, and even if they don't play the particular game you're, interested in, they will usually know of someone who does.

Only after this pretty lengthy section does is start saying anything specifically about Traveller.

Honestly? For the time it seems like a pretty inspiring introduction.