Enron basically started many decades ago as a natural gas producer and pipeline operator. The companies that merged to form Enron got into electric generation, natural gas distribution (basically operating natural gas utilities that provide gas to homes and businesses) and a few other side markets (like pulp and paper etc.)
Basically they were an “energy” company that operated pipeline (both oil and gas), power plants, and gas utilities. That part of their business is basically very stable and part of the “utilities industry” which is defined by heavy public regulation, public monopolies, and very stable but typically mediocre returns (versus the rest of the market.)
However around the time all the companies merged into Enron the guys at the top decided to move out of stable industries and started selling off large shares in their gas pipeline business and even eventually seemed to be moving away from any “hard” assets at all. The new money maker was basically buying and selling large amounts of energy as marketers, and running a huge website where companies from all over the country could enter into energy deals. This is a highly speculative business (but one that is definitely possible to make money in), where Enron started to really get insidious is they basically represented themselves as just and safe as stable as they were when they were mostly a utility company. They also started to use their utility assets to manipulate the market, as buyers and sellers of energy who also operated parts of the grid they could directly control supply. They would take power plants down for unnecessary maintenance to cause a short term surge in demand, and then sell energy at massive profit.
They also started to drive significant amounts of corporate profit based on long term energy deals in which they recorded all theoretical profit from the deal as present-day profits. So as said upthread, if price variations long term meant the deal might be bad, that wasn’t reflected on the present day balance sheet because they had marked the profits from the entire deal as already realized. What then happened is as energy prices fluctuated the actual money coming in was nothing like what they had “realized” already, and in some cases they were losing money on some of their long term deals.
This would have been revealed under ordinary accounting rules, but their independent auditor basically helped them fraudulently hide it from the market so investors kept treating Enron as a blue chip stock. When the curtain fell down the market reacted swiftly and Enron was bankrupt.