Yes, you're right about its meaning. An *Eselsbrücke* is, for instance, a word or phrase that helps you to remember something. A good English translation is *mnemonic*.

An example:

<pre>

    OSI-Modell:  
    <code><b>A</b>lle          <b>P</b>riester       <b>S</b>aufen    <b>T</b>equila     <b>N</b>ach      <b>D</b>er         <b>P</b>redigt.</code>
    Application / Presentation / Session / Transport / Network / Data Link / Phyiscal

</pre>  

About its origin, there's a quite possible version given at [redensarten.net][1].

The thing is that a horse (similar to a donkey in many respects) would simply jump over a ditch, a brook or whatever. The donkey, however, won't even go through the ditch.  
But if you build a bridge (or take an existing one), the donkey walks over that one without hesitation. <sup>1</sup>

The 'donkey bridge' helps to get to the other side.

In a [television show of ZDF][2], they second this story.


---

  <sup>1</sup> They mention (and the link in Matthias' comment seconds this) that the original meaning was quite a bit different and that it changed.  
However, I am not sure if there's an error in that article or if it's just a little too late for me right now, but I don't quite get it.

  [1]: http://www.redensarten.net/Eselsbruecke.html
  [2]: https://www.youtube.com/watch?v=VslhSTJ1FIQ