Tai is the superlative of size and so suitable English words might be Greatest, Supreme, Ultimate, Maximal and so forth. It is derived from the related character Ta which can be viewed as a person with arms out stretched to indicate some thing big - think fisherman arms apart to indicate a big fish. In Tai the Ta character is augmented by the small stroke below the wee man. This may be seen as indicating activity or energy - the fisherman is bouncing up and down in excitement with arms outstretched to indicate the enormity of the 'one that got away' :-)
Ji originally meant the ridge pole of a tent. The place where the sides come together on top. Later the idea of things coming together became generalised and it began to be used for road, path and trail intersections. The Shanghai railway marshalling yards with all the track are refered to as the Shanghai railway Ji.
Together they refer to the philosophical concept of the ultimate place, or point where 'everything becomes united'. I like to make an analogy with the idea of the still point in the centre of a tornado or the eye of a storm.
I hope that answers your question miguelin