Compared to .concat(), .merge() doesn't need to wait for the first Observable to complete before emitting values from the second one.

Let's take a look at this example:

Observable.merge(
        Observable.just(1L, 2L),
        Observable.just(3L, 4L)
)
        .subscribe(v -> log("subscribe", v));

It will produce the same output as .concat():

APP: subscribe:main:1
APP: subscribe:main:2
APP: subscribe:main:3
APP: subscribe:main:4

However, consider that we use the .interval() Observable as the first Observable, as illustrated:

Observable.merge(
        Observable.interval(3, TimeUnit.SECONDS),
        Observable.just(-1L, -2L)
)
        .subscribe(v -> log("subscribe", v));

The Subscription will then produce values from both the Observables just fine:

APP: subscribe:main:-1
APP: subscribe:main:-2
APP: subscribe:RxComputationThreadPool-1:0
APP: subscribe:RxComputationThreadPool-1:1

This is because .merge() doesn't wait for the first Observable to complete before emitting more values.