Otoshi(or tsukidashi in Osaka) is an appetizer which is served at izakaya style restaurant in Japan. Usually the staff of the restaurant will bring it to you right after you sat down or when he/she brings you your drink. This appetizer is a very small portion and unfortunately not free. It will be charge in lieu of cover charge. It is similar to couvert in Czech Republic which restaurants charges for bread, ketchup, or accompaniments even you have not ordered them. Otoshi is usually around JPY 300 but you may refuse it when the staff brings it to you. However, you should inform the staff immediately when you sat down.
One thing you have to watch out is that there is no explanation of the otoshi written in the menu card due to it is different each day. Then, usually the staff will not explain what is the today's otoshi. Therefore if you have certain problems with the ingredients, better to inform the staff in advance.