The quick answer is that Tandem works side by side with Slack (or any other instant messaging app)
With Tandem you can confidently know if your call will be answered. Slack show your teammates as grey or green. In Tandem you can confidently know if you can call the other person with an assured answer. Tandem automatically change status from green (available) to orange (away) when the mouse or keyboard haven't been used.
Talking in Tandem is instant, like tapping someone on the shoulder. When you click on a teammates name you'll start an immediate conversation with the other person.
With Tandem you can join your teammates work apps and collaborate at new level. When you're talking with your teammate you can see which work app they're actively working on and jump into them.
We integrate with Slack tightly and most teams use Tandem together with Slack.