You can do so much with Discord! However, it is primarily a communications platform. You can interact with large groups of like-minded people in servers, or have one-on-one conversations with your friends.
You may have used an app like Discord before. If you've used chat services for playing games with your friends before, you may have used TeamSpeak or Skype. If you've worked in an office, you may have used Slack. All of these services are similar to Discord, and having used any of them will give you a great start to using Discord!
Discord is not just a basic communications platform. For one thing, it plays well with gamers because it has very little impact on your system when you're playing games. It's also not just a place to talk to your team or best friends. It's a social media platform to meet new people on. Joining a server based on your interests is a gateway to meeting a ton of people who share that interest.
You don't even have to join a server if you don't want to. You can make your own server about whatever you want. It only takes a moment to set up a server. Give it a name, upload an optional photo, and then invite your friends!
Discord has easily-accessible text chat, which you may be used to from any chat or instant message platform. But Discord is so much more than that. You can easily use voice chat or video chat, if you have the appropriate hardware. Or, even if you just have a smartphone, since Discord has a robust mobile app. While video chat is only available in a private message, voice chat is accessible in special server channels. This is great for playing games and squading up with fellow server members.