From High Roller Guides

The match percentage is the percentage of free bonus money you will receive relative to your deposit.

For example: If a casino has a match percentage of 200% that means if you deposit $100 you will get an additional $200 bonus money.

$100 * 200% = $200

Usually there are rules for the amount of money you must risk in order to cash out, so you cannot simply deposit, and then cash out your bonus money.