What do these terms mean? (Glossary)

Download:
How quickly you can pull data from a server on the internet to your device. Most connections are designed to download much faster than they upload, since the majority of online activity, like loading web pages or streaming videos, consists of downloads.
Upload:
How quickly you send data from your device to the internet. A fast upload speed is helpful when sending large files via email, or in using video-chat to talk to someone else online (since you have to send your video feed to them).
Ping:
Also called latency, ping is the reaction time of your connection–how quickly your device gets a response after you’ve sent out a request. A fast ping means a more responsive connection, especially in applications where timing is everything (like video games). Ping is measured in milliseconds (ms).
Packet loss:
Packet loss occurs when a packet of data being sent over the internet is not received or is incomplete. This is described in percentage of packets lost compared to packets sent. Packet loss in most cases is result of poor signal/line quality. Packet loss testing is available with Speedtest desktop apps.
Jitter:
Also called Packet Delay Variation (PDV), jitter frequency is a measure of the variability in ping over time. Jitter is not usually noticeable when reading text, but when streaming and gaming a high jitter can result in buffering and other interruptions. Technically, this is a measure of the average of the deviation from the mean. Jitter testing is available with Speedtest desktop apps.
Mbps:
egabits per second. A megabit is 1 million bits of information. This is a standard measure of internet speed, not to be confused with megabytes (MB) which is a measure of size rather than bandwidth.
Kbps:
Kilobits per second. A kilobit is 1,000 bits of information. This older measure of internet speed is used only when needed to describe slower connections, and not to be confused with kilobytes (KB) which is a measure of size rather than bandwidth.