What exactly is a transaction in APM?

A transaction is a particular route that you support in your back-end. Note that a route is a particular endpoint that your client requests, which is a combination of a URI (or path) and a specific HTTP request method (GET, POST, and so on).

GET /user/:id is the transaction in the below route example

copy

router.get("/user/:id", function(req, res) {
    res.json({ "message" : "Hello " + req.params.id });
});