Kosher food is a diet Jews believe Yahweh commands them to adhere to. Food that's acceptable to Muslims is called Halal. Neither Jews not Muslims can eat pork without believing that they have sinned. Kosher meat must come from animals that have been ritually slaughtered. There are complaints about Jewish ritual slaughter but animals slaughtered that way have a less stressful journey to the slaughterhouse. They are packed less densely to prevent injuries that would stop them being acceptable as Kosher. Kosher meat can only come from animals that divide the hoof and chew cud. Fish must have scales and fins. A few species of birds are acceptable. Milk and meat may not be eaten at the same meal. There are very many other complicated rules. Even observant Jews sometimes have to ask a Rabbi in difficult cases. A few Kosher restaurants employ a Rabbi to supervise the kitchen staff and ensure that no rule is unknowingly broken.
They do this to please their god, but it's not clear what it does. Either way it's silly that people won't eat certain things to please their gods.
Kosher food includes the meat (salted to remove blood) of goats, cows, chicken, and so on; it doesn't include the meat of dolphins or whales ("fish" without scales), pigs, crustaceans, insects, and so on. Virtually every vegetable, fruit, or other plant may be eaten, but genetically modified foods and cross-bred food are seen as 'Shaatnez', mixed, foods and so are forbidden.
The term 'Kosher' is also extended to cover any conduct in accordance with the Halacha, the body of Jewish religious law. For example, a napkin might not be 'kosher' if Jews were employed on a Shabbat to manufacture it, so a napkin pack might be marked as 'kosher'.