“401k” is an American term of art. It’s like a pension fund except you’re directly investing into the stock market and not pooling risk with anyone else. Money contributed to a 401k isn’t taxed until you retire, but in exchange you can only contribute direct earnings from the job sponsoring your account.
As part of a benefits package, some employers also offer contribution “matching”. It’s very similar to the concept of employers matching charitable donations – for every personal dollar you put in, they chip in as well. How much they contribute will also vary. Some places will do dollar-for-dollar matching up to a maximum salary percentage (e.g.: If I earn $50k and get 5% matching, the employer will match the first $2500 I contribute). Other companies will instead contribute pennies on the dollar at a fixed percentage rate (e.g.: If I save the annual maximum of $22,500 and get 5% matching, the employer will contribute $1,125). And yes – it’s never a pleasant surprise when you’re expecting the good matching and instead get the shitty matching.
In any case, because 401k matching is technically only a job benefit, there aren’t many rules against employers reneging on it. It’s one of the first corners that tend to get cut in workplaces where the boss doesn’t have to look their underlings in the eye on a regular basis.