Truth consists in the equation of mind and thing, as said above (Question 16, Article 1). Now the mind, that is the cause of the thing, is related to it as its rule and measure; whereas the converse is the case with the mind that receives its knowledge from things. When therefore things are the measure and rule of the mind, truth consists in the equation of the mind to the thing, as happens in ourselves. For according as a thing is, or is not, our thoughts or our words about it are true or false. But when the mind is the rule or measure of things, truth consists in the equation of the thing to the mind; just as the work of an artist is said to be true, when it is in accordance with his art.
St. Thomas is talking about something he elaborates on elsewhere, namely that reality is the measure of truth. Our minds know the truth to the extent that they know the way things really are — pretty much a tautology.
For God it’s the other way around: Things are the way they are because he knows them.
He is the measure of them; they are the measure of us. His knowing makes them be; their being makes us know.