what does the following tr command do?

I am looking at a .sh script and one of the lines is the following:

tr -d '\15'  /testProject/files/file1.txt

i understood the part file1.txt > /testProject/files/file1.txt which means to add the result to /testProject/files/file1.txt but what does tr -d '\15' < file1.txt do? I know \15 is an ASCAII character but not sure what it does. Would really appreciate if someone could help.