i heard that Jewish people can only eat kosher food, why iis that? and how is a food a kosher food :*)
Kosher foods are those that conform to the regulations of the Jewish Halakhic law framework, kosher meaning fit or allowed to be eaten. A list of some kosher foods are found in the book of Leviticus 11:1-47. There are also certain kosher rules found there. Reasons for food not being kosher include the presence of ingredients derived from nonkosher animals or from kosher animals that were not properly slaughtered, a mixture of meat and milk, wine or grape juice (or their derivatives) produced without supervision, the use of produce from Israel that has not been tithed, or the use of nonkosher cooking utensils and machinery.
Kosher is the Hebrew term for eating according to the Scriptures...but they also added a bunch of man made rules that are not applicable to the Bible. Which are you interested in? We eat Kosher according to the Bible, and it is VERY healthy and have lots of benefits!
Btw, the issue with produce from Israel is no longer about tithing, since their is no Temple priesthood to collect tithes. It is, rather, about produce cultivated in the shmita year of rest for the land that comes every seven years.
Since the word literally means fit or proper, as blakbeltv3 pointed out, it is often used of things other than food (for example, the book "Kosher Sex" by Shmuley Boteach), but then its meaning is looser and not so technical.