Galatians is a letter to the church in the area of Galatia (modern day Turkey). Like many other churches at the time, Galatia had been influenced by false teachers who were saying that all Christians, Jewish or Gentile, had to obey Jewish laws, such as not eating certain kinds of meat. Paul wrote this fiery letter to put them straight. He displayed his anger that the way of freedom that Jesus came to bring had been turned back to a message of law-keeping.
Galatians really drives it home that we are not saved (or in a right relationship with God) because we follow rules or laws, but only because of the grace and love of Jesus Christ. It's a big reminder that we are saved by grace, not by anything we do - and that's so freeing!