1.
In the US, education is not standardized. What happens in one school may be different than what happens in a school in the same city. And both of those can be very different than another school 20 miles away. "Finals" in one school can be different than "finals" in a different school.
2.
In many schools, students take classes that last for the entire school year*. When that happens, the teachers will often give tests and assignments on a regular basis that cover what was just learned in class. (e.g., the first test will just cover chapter 1-3 of the book. the second test will just be about the things in chapter 4 (no questions about chapters 1-3).) At the end of the year, the teacher will give a "final exam" that covers everything that the student was supposed to have learned throughout the year. Many teachers will make the final exam the most important part of the grade and a more difficult exam than the earlier assignments and tests.
Because high school students often take between 4 and 8 classes and college students usually take between 3 and 6 (again, depending on a lot of things) and the school year ends for all of their classes at the same time, the students will take a number of difficult tests all in the same two or three days.
"Finals" can refer to the tests themselves or the period of time when the test are being given (e.g., "finals week")
* some schools have semesters, some have trimesters, some have quarters, some have 6-week periods. Every school can do whatever it wants.