what does "Te extrano!!!" mean?
Hello, and welcome to the forum. It means, "I miss you." And, just FYI, your friend didn't write it quite properly. It should read te extraño.
Have a look at this, it will answer many of your questions
Hiiii Te extrano- i miss you example- in spanish - te extrano,hijo, siempre pienso en ti; in english - i miss you,son, i am always thinking about you; okey