A ladder is a fairly common Masonic symbol. It appears on many Entered Apprentice tracing boards. Often it refers to the ladder that Jacob saw in a dream and which was used by angels to travel up and down. This image is from a second degree Adoption tracing board, but there is also a third degree Adoption tracing board with this image.
The ladder is an important symbol in Adoption Freemasonry. A candidate is even received: “Between the Tower of Babel and Jacob’s Ladder, and at the foot of Noah’s Ark.”
There are common, but also more elaborate explanations for the ladder in Adoption rituals.
This ladder is mysterious. It symbolises the invisible and incessant relationships between heaven and earth and vice versa. The foot of the ladder rests on the earth and the top reaches beyond the clouds. (Manuel Complet de la Maçonnerie d’Adoption, J.M. Ragon, 1860)
But also:
This ladder is all mysterious, the two uprights represent the love of God and neighbour, and the rungs the virtues that derive from a beautiful soul. (Adoption ou la Maçonnerie des Dames, 1783)
Ragon (1860) ads:
This ladder seems to be a reminiscence of the one that the Persians the mysteries of Mithras, to explain to the neophytes the double the double movement of the fixed stars and the planets, their continuous relations with the earth and, reciprocally, those of the earth with these stars, for the perpetual exchange of their mutual emanations and, finally, to represent the passage of souls through the celestial spheres the belief of the time. Along this Mithraic ladder were 7 doors the first, made of lead, designated Saturn; the 2nd, made of tin, Venus; the 3rd, made of copper copper, Jupiter; 4º, of various metals, Mercury; 5º, of iron, Mars; 6º, of silver, the moon; 7º, of gold, the sun; then the empyrean sky. There you have it, without a doubt, the scale of Jacob’s dream, the allegorical idea of which existed among the Chaldeans, the Persians and the Egyptians, many centuries before Abraham, Isaac and Jacob, which M. Vincent, in his book on Idolatry Idolatry (Paris, 1850): the 1st, the Most High, Uranus Brahma; the 2nd, the Power of Power of harmony; the 3rd, the Measurer of light, the Sun.
Also see Tower with Ladder.
Can be found in Feddersen (Die Arbeitstafel in der Freimaurerei Band I (1982) F/54, P. 324). He gives as source La Franc-Maçonne ou Relevation des Mysteres des Francs-Maçons 1744. It is not in my copy of that text though, but it is printed in Adoption ou la Maçonnerie des Dames from 1783.