Flux is generated when releasing Essentia into the world without filtering it. Flux is sort of a bad aura around a chunk. Too much flux can cause less vis to generate in the area and can also lead to rifts. Rifts will destroy any blocks that come in contact with them. Over time these rifts will decrease the amount of flux in a chunk. After a while these rifts will leave behind a giant taint seed. These giant taint seeds spread and maintain Taint. This is almost the only way to obtain Vitium, which is required for Essentia Tubes (even though you can get vitium from mycelium, nether wart and vishrooms).
Flux rifts generate on the highest block in the corrupted chunk.
These rifts have a chance of having any of the following happen:
The size of these rifts depends on the amount of flux that has built up in the area.
In order to close rifts you will need a Causality Collapser but for now if you do find one or create one then I would suggest you move away or kill anything that spawns from it.
You can stabilize rifts with a Stabilizer, but it creates flux.
If there are crystal clusters growing in the fluxed area, there is a chance they can turn into Flux Crystals. This process consumes a little bit of flux from the aura during this process.
Flux can be evenly distributed between two chunks (maybe even 3) by placing the essentia smelter directly in the corner of a chunk and putting Auxiliary Venting Ports directly next to the chunk-border. The Flux will be calculated in the Chunk to the output of the Auxiliary Venting Port. This helps to let the Flux be minimized per chunk.