What is the meaning of [One swallow doesn’t make a summer]

A single instance of something is just that; it doesn’t indicate a trend.

This old proverb is listed in several early glossaries, notably Richard Taverner’s transcription of the [Latin] proverbs of Erasmus – Prouerbes or adagies with newe addicions, gathered out of the Chiliades of Erasmus, 1539:

It is not one swalowe that bryngeth in somer. It is not one good qualitie that maketh a man good.