Do this tutorial and you will see - https://alankent.me/2015/07/24/creating-a-new-rest-web-service-in-magento-2/
Specifically follow closely where the array of float is passed through in the body of the HTTP request.
app/code/AlanKent/CalculatorWebService/etc/webapi.xml
<!-- Example: curl http://127.0.0.1/index.php/rest/V1/calculator/add/1/2 --> <route url="/V1/calculator/add/:num1/:num2" method="GET"> <service class="AlanKent\CalculatorWebService\Api\CalculatorInterface" method="add"/> <resources> <resource ref="anonymous"/> </resources> </route> <!-- Example: curl -d '{"nums":[1.1,2.2,3.3]}' -H 'Content-Type: application/json' http://127.0.0.1/index.php/rest/V1/calculator/sum --> <route url="/V1/calculator/sum" method="POST"> <service class="AlanKent\CalculatorWebService\Api\CalculatorInterface" method="sum"/> <resources> <resource ref="anonymous"/> </resources> </route>