I did some of this back in the States.

A merchandiser ensures product displays are correct, stocked, and in the right location in the store. (Some retailers pay for better placement.)

Sometimes merchandisers reset aisles of product in a store. Not sure about China.

Sometimes merchandisers work in the store during weekends and other busy times giving samples to customers and talking about the product. 

Fun work IMHO. Kept me busy for almost a year.

It may differ here in China. Ask them!

Why do you ask? Someone looking for a merchandiser? 

Here you are: http://www.lmgtfy.com/?q=what+is+a+merchandiser%3F  Do Enjoy!

It's a stand that you put things on for sale, maybe even a rack, but sometimes the merchandise comes with a rack. The bigger the rack, the better the product.

Victoria - you have thorough answers above (bill certainly) but for reference if you have any confusion about English words a faster way might be to google (rather than baidu - I assume you normally use that search engine) the word and read the first the 3-4 answers.  Pay particular attention to anything in wikipedia since that's generally (but not always) more reliable than other sources.  And ignore the paid ads at the top of the search results - you want a match for your search after all, not promotional junk.

e.g. you could go to google.com and try "merchandiser definition"
Last update: it might be necessary to click on the "English" version of google to get results written by English native speakers since it's now redirected through hong kong.