Problem description & analysis:
The table below has two columns, where column B is the number.

We need to repeat the value of column A N times according to the value of column B and concatenate the results into one column.

Solution:
Use SPL XLL to do this:

=spl("=?.conj(~2*[~1])",A1:B2)

As shown in the picture below:

Explanation:
conj() function concatenates members of a sequence; ~2 represents the 2nd child member of the current member; “integer N* sequence” means copying each member of a sequence N times.