What's the meaning of "theme()"? I am new to drupal, I read some introductory tutorial. 

I notice there is some theme() function in template files.

Like this:

&lt;?php print theme('uc_price',array('price'=&gt;$price)); ?&gt;


It will generate some html source code:

&lt;span class="uc-price" data-thmr="thmr_45"&gt;$59.85&lt;/span&gt;


Is it theme() function is only to generate the html source? If this is the case, why I need use theme() function? I Think just only write html source manually.

I use Zen base-theme, Does theme() is one of zen's native function?

Please tell me the real usage of "theme()".
Your understanding about theme is right. It is for wrapping data with html elements. Advantage is that you can seperate view part and logic part. There are also default theme which you can use out of box