–verb (used with object), -at·ed, -at·ing.
to incite or move to action; impel; motivate: actuated by selfish motives.
2. to put into action; start a process; turn on: to actuate a machine.
You pretty much have it. A shutter "actuation" is the opening and closing of the shutter when a picture is taken. It should be noted that you can actuate the shutter for other purposes, most notably to enable sensor cleaning in many modern DSLR cameras. The total actuation count of a camera may be more than the total number of pictures it has taken, so it is important to differentiate between the number of photos and the number of actuations.
Generally speaking, I personally find shutters that have less than 50,000 actuations have too few. On average, I figure I take 500-700 pictures a week when I am active (I've had a couple inactive periods this year, so I'm a bit under that for the year.) In a single year, on average, that would be about 30,000 shutter actuations. A camera with a shutter rating of 50,000 actuations would probably last me two years, maybe three. For a low-end DSLR, three years is probably not too bad, but it wouldn't hold up under professional use. A pro could very likely burn through 50,000 actuations in a year or less, under much more strenuous situations. Shutter lifetimes of 300,000 actuations would probably hold up for 3-4 years of professional use in the real world. Many low- and mid-range DSLR's these days have shutter lifetimes of 100,000 to 150,000 actuations, and under normal usage, a camera body should last for quite a number of years.
The type of photography you do can have an effect on shutter lifetime. If you are an avid sports photographer that makes extensive use of burst mode on your camera and takes several thousand shots a week, a 300,000 actuation shutter may only last 2-3 years. On the other hand, if you are more the landscape photography type, who may snap a photo once every few minutes, and only a few at a time, a 150,000 actuation shutter may last you for a decade.