A Yod is a combination of astrological aspects between 3 planets where two astral items A and B are in sextile, and a third planet C, called the apex, is in quincunx with both A and B. It tells you where you should focus your attention or where your resources are.
A Yod transit is a sharp astrological request, but not an obligation, to connect dots in a situation and to take action using available opportunities. Say you have Chiron sextile Moon, and Jupiter is quincunx with both Chiron and Moon. The energies of Jupiter are currently not connected with those of your Moon and Chiron. You may be involved in an expansive life project, but somehow, it may have consumed all your energy and motivation. You have reached your limits, you are exhausted. You need to regroup. But, it is not on your priority list. You ignore that the energies of Chiron, the wounded healer, and those of your Moon, to rebuild a sense of cocooning, are available. You could use them.
The Yod is the "Finger of God" pointing your attention at the issue: C (Jupiter). It tells you where it would be wise to spend your time and where your resources are (A and B). It may come as a light bulb moment, a snap, a sparkle or a breakdown.
In a birth chart, A and B are available to support and to guide the apex, although the bearer has to learn that these exist and how to use them to develop the full potential of the apex.
In synastry, if a partner has an astral item in the apex and the other has both in A and B, it suggest that partner AB could be a strong support and great guidance for the other partner. It is an opportunity.
Related aspects: sextile and quincunx & inconjunct
Other minor & rare aspects: grand-cross, grand trine, quintile & biquintile, semi-sextile, semi-square, sesquisquare and t-square