There are partial holes in at at one end. You insert a small amount of dyed DNA (etc) containing solution each. Apply an electrical potential across the gel. DNA gradually moves along. Smaller DNA fragments move faster. So, at a given time, you can coarsely measure fragment size of a given sample. Your absolute scale is given by "standards", aka "ladders" that have samples of multiple, known sizes.
The paper authors cheated (allegedly) by copy + pasting images of the gel. This is what was caught, so it implies they may have made up some or all results in this and other papers.
The problem are the vertical labels
In Figure 1e it says: "MT1+2", "MT2" and "MT1"
In Figure 3a it says: "5'-CR1", "CR2" and "3'-UTR"
In Figure 3b it says: "CR2", "CR3" and "CR4"
Most people only remember the initial publication and the noise it makes. The updated/retractions generally are not remembered resulting in the same "generally, no consequences" but the details matter
In my area we have a few research groups that are very trustworthy and it's safe to try to combine their result with one of our ideas to get a new result. Other groups have a mixed history of dubious results, they don't lie but they cherry pick too much, so their result may not be generalizable to use as a foundation for our research.
[1] Exact reproduction are difficult to publish, but if you reproduce a result and make a twist, it may be good enough to be published.
(And I think part of the general blowback against the credibility of science amongst the public is because there's been a big emphasis in popular communication that "peer reviewed paper == credible", which is an important distortion from the real message "peer reviewed paper is the minimum bar for credible", and high-profile cases of incorrect results or fraud are obvious problems with the first statement)
Also, many sites just copy&paste the press release from the university that many times has a lot of exaggerations, and sometimes they ad a few more.
[1] If the journal has too many single author articles, it's a big red flag.
Jan Hendrick Schon (he was even stripped of his Phd, which is not possible in most jurisdictions) He made up over 200 papers about organic semiconductors
Victor Ninov who lied about creating like 4 different elements
Hwang Woo-suk who faked cloning humans and other mammals, lied about the completely unethical acquisition of human egg cells, and literally had the entire Korean government attempting to prevent him from being discredited, and was caught primarily because his papers were reusing pictures of cells. Hilariously, his lab successfully cloned a dog which was considered difficult at the time.
Pons and Fleischmann didn't do any actual fraud. They were merely startlingly incompetent, incurious, and arrogant. They still never did real research again.