In simple terms, Applymap works is a lot like a vlookup in Excel, but in two stages. First you use mapping load to load the table with an identifier column and the labels. Then, in your destination table, you call the function applymap so it returns the label based on a field you have in that table, like this:
applymap('name of the mapping table',Field with values to identifier column,value if no match is found) // qlikview help has more detail
A left join can also be used instead, and it's almost identical. But be aware that a left join will create duplicates if the data you want to join isn't unique. Applymap is good here because it just links one value.