Jellyfish, 12 bytes

P#;#'*
 E 10

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Explanation

Using more conventional notation, this program represents the following expression:

P( #(10, ;(#(10, '*))))

# is repetition, so the innermost call just creates a string of 10 asterisks. ; wraps the result in a singleton array. Then we have another repetition to create 10 copies of the string. P prints the array in "matrix format" which prints each string on its own line.

Jellyfish, 12 10 bytes

Thanks to Zgarb for saving 2 bytes.

P$'*
 &;10

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Explanation

Using more conventional notation, this program represents the following expression:

P( $( &;(10), '* ) )

&; takes a single value and creates a pair with two times that value, so &;(10) gives us [10 10]. Then $ is reshape which forms a 10x10 grid of asterisks. Finally, P prints the array in "matrix format" which prints each string on its own line.