In Short, a speed test will test your Internet speed to a test server by downloading and uploading Data packets, to and from a test server.
Speed tests can be used as an indicator but are not always accurate and can be affected by how busy the test server may be or by the performance of the devices the tests are done from (Laptop or phone).
The Download speeds indicate how fast your connection can receive data from the test server. Most of the Internet data is downloaded from websites or servers. Download speeds are measured in megabits per second (Mbps).
The upload speeds indicate how fast your connection can sent data to the test server. Uploading is necessary to send requests to the internet and sending files or doing a video chat. Upload speeds are also measured in megabits per second (Mbps).
The Ping will indicate the reaction time (latency) of your connection, meaning how fast you get a response from a request. Bad pings can affect voice or video calls and are measured in milliseconds (ms).