Ping measures the minimum time needed for your computer to send a 'hello' to another computer and receive a response and is measured in milliseconds (ms) where 1000ms = 1 second.
This is the information coming from the Internet to you, like a video you watch or email you receive and is measured in Megabits per second (Mbps). If your plan is 100/40 you should get at least 80% of the 100 - so 80Mbps would be a good download speed.
Speed tests move data towards and away from your device to another computer called a Server. To get the best test results, test servers are normally close to you and really fast.
You can choose a different server to see what the difference is, but normally let it choose the best one for the most accurate results
A Ping is an electric pulse sent over your cabling across the Internet. The path it takes is not always the same, so times do vary and this is overall quality is called Jitter. Low Jitter is a sign good quality cabling and is essential for video streaming quality, Internet games and VoIP phone calls.
Anything you send is going to the Internet, which could be an email you send, what someone lese sees in a video chat or your voice in a phone call.
If your plan is 50/25 you should get at least 80% of the 25 - so 20Mbps would be a good upload speed.
All speed tests show a single moment-in-time test and can vary a lot. Tests at 8am will be faster as the Internet (and mostly your ISP) have much less users.
Between 3-6pm and 8-11pm the Internet gets congested with kids playing online games and watching Netflix so expect your tests to be even 1/2 as fast.