The md5.txt files contain the MD5 values for files in that directory. Values are added to it when file system images are imported into the system or when Autopsy creates the file. This mode allows one to calculate the MD5 value if it was not created before and to validate the integrity.
When the Image Integrity button is selected from the Host Manager window, all files will be shown with their MD5 value if known. Here any image can have its value verified or a new one created.