It is an easy to understand aspect. The opposition between A and B means balance has to be restored between both energies, typically through adjustments. If not, things may get out of control and the flow of energy will be suboptimal (excessive or leading to lethargy). Oppositions are also about projections. They can suggest a belief that the solution to one's issues will come from someone external (or external factors), while in fact, we are the ones who have to learn to adjust.
Oppositions can remain unresolved for years. It can take a lot of life experience before we understand how they impact our lives and decide to do something about it. But, with a t-square, the third astral item C is putting a lot of pressure to resolve imbalances between A and B. The longer the transit, the harder the pressure. You are well awareness that something needs to be fixed, if it has not already been fixed yet.
In a birth chart, it is somehow different. Say you have Moon opposition Venus, and both are square with Jupiter. You inner emotions are not easily in sync with the ones you experience with your partners. You don't exactly know how to connect and express them. Jupiter may help with its tolerance, benevolence and sense of well-being. But sometimes, it can make things worse through exuberance or excessive ideals and hopes in love. Native chart t-squares do not create out of control and beyond repair situations alone, but they never create stable and inert situations too.
T-square in birth charts do not go unnoticed by those surrounding the bearer. They automatically emerge as character traits and idiosyncracies. In synastry, if one partner (or friend) has an astral item in C, it will intervene to regulate the other partner having A and B in opposition. But, when one partner has a square, say, between A and C, and the other partner has an astral item in B, don't be surprised if partner AC is developing push-pull attitudes towards partner B. It may not be obvious at the beginning, but the longer they stay together, the more obvious it will be.
The good thing about t-squares, is that we are always aware of the energies at play. Therefore, we are in a position to act and to take control. We can do something about it. And, we also know the price we will have to pay when we don't take action. The more we have adjusted to solve the AB opposition, the less the t-square will stress us to resolve it.
About the missing leg interpretationSome astrologers argue that in order to smooth the challenging issues resulting from t-squares, you need to take a look at a missing leg which would be opposite to the C item. This is where energy would be available for help. The flip side of this approach is that it's a recipe to provoke the occurence of a grand cross, and these are much more challenging than t-squares. However, sometimes, the amount of energy of a grand-cross is required trigger the kind of major shake-up required to smash everything that does not work before one can start with a blank page again. It is a really though medecine and should be used with caution.
Related aspects: square, sesquisquare, semi-square and grand cross
Other minor & rare aspects: grand-cross, , grand trine, quincunx & inconjunct, quintile & biquintile, semi-sextile and yod