Beginnings are always delightful; the threshold is the place to pause.
What is its meaning?
Interpretation of literature is off-topic on this website, and it seems very likely to me that the people who quoted this sentence of Goethe in translation may have intended a meaning quite different from what Goethe intended in the original.
The actual Goethe quote goes:
“Here is your indenture,” said the Abbé: “take it to heart; it is of weighty import.” Wilhelm lifted, opened it, and read:
Art is long, life short, judgment difficult, opportunity transient. To act is easy, to think is hard; to act according to our thought is troublesome. Every beginning is cheerful; the threshold is the place of expectation. The boy stands astonished, his impressions guide him; he learns sportfully, seriousness comes on him by surprise. Imitation is born with us; what should be imitated is not easy to discover. The excellent is rarely found, more rarely valued. The height charms us, the steps to it do not: with the summit in our eye, we love to walk along the plain. …
Die Kunst ist lang, das Leben kurz, das Urteil schwierig, die Gelegenheit flüchtig. Handeln ist leicht, Denken schwer; nach dem Gedanken handeln unbequem. Aller Anfang ist heiter, die Schwelle ist der Platz der Erwartung.
Let me make a few observations.
"place to pause" and "place of expectation" mean quite different things, and "place of expectation" is a much more accurate translation.
This whole passage conveys the idea that starting something is easier than finishing it.
This is reminiscent of Polonius's speech in Hamlet, in that both are collections of \platitudes. Maybe it was not meant to be taken too seriously.
If you Google "aller Anfang ist", you get many, many hits for "aller Anfang ist schwer", meaning "every beginning is difficult." I presume that this adage was around when Goethe wrote this, and he is playing off it.
I take it to mean enjoy what is happening in the seminal moment because it's all downhill from there.
This can apply to falling in love, to marriage, to anything that we enjoy in the beginning and soon will change. Therefore pause and enjoy that moment in time, because once we cross the threshold, change is inevitable.
In the case of the episode of Poirot. It foreshadows an impending murder at what appears to be the beginning a very enjoyable party.
The first sentence appears easy to be understood: the beginnings of doing something is always delightful. In many cases we start to do something because we can reasonably expect a delightful effect if we start to do it. The second sentence is the key idea hidden behind. I think the second sentence holds two meanings.
One is that when the delightful effect or feeling significantly decreases, we arrive/reach the threshold (in other words, critical point) of the expectation of final results of what we are doing. This implies that a considerable change of the activity in the subsequent procedure or in strategy should be taken into consideration.
The other meaning is like a sign of predicting an occurrence of an unhappen thing. Sometimes we are not sensitive or aware of that something which will definitely change the delightful feeling (as it happens in the beginning). The second meaning of the second sentence is to suggest that when we meat a pause, it possibly implies a threshold (the point that something will happen) that will considerably change of the previously probably continuous delightful feeling.