An assessor is a government position meant to evaluate an approximate value of the properties in their jurisdiction to calculate future taxes.
These evaluations are usually done annually, or every time a significant upgrade has been done to the property. There are some cases where assessments are only needed to be done once.
Assessors do this by considering the market's current average value and assigning a dollar amount to the property. Then it becomes easier to calculate the fair market value.
In most cases, assessors need to be elected for this position and have a basic certification.