Tantra is a vast subject. Scholars are still unsure if tantra arose from yoga or yoga developed from tantra. In either case, it was part of a world-wide recognition over several centuries of a unitary reality higher than the currents of everyday life. Polytheism gave way to something higher. In the West monotheism flourished, in the East the concept of a unified reality was laid on top of, and subsumed, a patchwork of religious deities.
If you work away the cultural baggage and elaboration, tantra has a few simple things to say:
This is it; the world is perfectly real
all of reality is vibrating (spanda) and conscious
humans are never disconnected from the universe
you can improve the connection greatly.
What becomes complicated is all the ways you can improve the connection, because there is no end to the variations in humans. Tantrics/yogis have spent centuries exploring what George Feuerstein in The Yoga Tradition called psychotechnologies, techniques of consciousness. They developed thousands of visualizations of the energy centers (chakras) and energy flow of the body, and techniques for altering the flow.
Feuerstein also suggests that Tantra has a very body-centered orientation. Others have suggested that all religion and mysticism is body-centered, because humans unconsciously seek to understand the universe through the body they live in.
Either way, Tantra is all about the body. From elaborate visualizations of cutting off all your limbs in order to subdue your senses to the most sensuous raising of energy.
Tantra sees everything in the universe vibrating with energy, in both separateness and wholeness. Tantra knows the universe is always flowing and we must match our perception to each flow. Matching our perception to the flow means using the best tools we have, many of which are scientific modern perspectives.