I was shown how to create a Vlookup, linking pages, but have forgotton the principles of how it works?

VLOOKUP
Searches for a value in the leftmost column of a table, and then returns a value in the same row from a column you specify in the table.

The V in VLOOKUP stands for "Vertical."

Syntax

VLOOKUP(lookup_value,table_array,col_index_num,range_lookup)

Lookup_value:
is the value to be found in the first column of the array. Lookup_value can be a value, a reference, or a text string.

Table_array:
is the table of information in which data is looked up. Use a reference to a range or a range name, such as Database or List.

If range_lookup is TRUE, the values in the first column of table_array must be placed in ascending order: ..., -2, -1, 0, 1, 2, ..., A-Z, FALSE, TRUE; otherwise VLOOKUP may not give the correct value. If range_lookup is FALSE, table_array does not need to be sorted.
You can put the values in ascending order by choosing the Sort command from the Data menu and selecting Ascending.

Col_index_num:
is the column number in table_array from which the matching value must be returned. A col_index_num of 1 returns the value in the first column in table_array; a col_index_num of 2 returns the value in the second column in table_array, and so on. If col_index_num is less than 1, VLOOKUP returns the #VALUE! error value; if col_index_num is greater than the number of columns in table_array, VLOOKUP returns the #REF! error value.

Range_lookup:
is a logical value that specifies whether you want VLOOKUP to find an exact match or an approximate match. If TRUE or omitted, an approximate match is returned. In other words, if an exact match is not found, the next largest value that is less than lookup_value is returned. If FALSE, VLOOKUP will find an exact match. If one is not found, the error value #N/A is returned.