What is an appraiser?
An appraiser is a person or an organization that specializes in assessing the proper monetary value of an asset.
For example, it might be difficult for you to know the actual value of a house you are buying if you are not a real estate professional.
For this reason, you can bring in an unbiased third party to evaluate the house for you before you hand your money to the seller.
The appraiser will give you his opinion on the proper value of the house based on location, data from the house, market, etc. You can then decide to buy the house or change your mind depending on his findings.