What does it mean when colleges send you letters?

Im a sopre in high school and i took the PSAT earlier this year. Starting around 3 weeks ago i started getting about 10-15 letters from colleges a day. They said that they got my name from the college board from when i took the PSAT. Most of the schools were normal colleges, but others are exceptionally well-known and prestigious colleges. Some of the letters i got were from really good schools. Some of the ones that i can remember were Duke,NYU,Columbia, Brown, and the University of Chicago. I am very interested into atendin some of these schools and others similiar to them. I got pretty good scores on the PSAT and im ranked 8 out about 430 in a pretty good catholic school and am taking all rs/ap cles. By getting letters from these school, does that mean that i have a good shot of getting in? I also have talked to other ppl who have gotten letters as well and no one who have talked to has gotten a letter from schools of the caliber that i mentioned above.