The Alexandrine is a verse of iambic hexameter. No more, no less. One line in iambic hexameter. That means that in this line there are six feet, in which the second step in these feet are stressed.

taDum/taDum/taDum/taDum/taDum/taDum/ 

The Alexandrine finds it's origin in France and is quite rare in English.  Alexandrines are used as a variation on the iambic pentameter - a good example is the Spenserian Stanza, in which the last line is an Alexandrine.

An Example:

Smile, it makes  people  wonder  what you're  up to!

(Smile/it makes/people/wonder/what  you're/up to)