The burned tapes were a handful of video cassettes that Jay literally stumbled upon in Entry #69 when he tripped over a hole near the red tower. Jay and Tim believe that they are tapes Alex tried to destroy by burning before handing over the Marble Hornets production tapes to Jay in 2006. Footage in Entry #71 demonstrates that while Alex definitely set aside these tapes prior to giving the production tapes to Jay, but most likely did not burn them until later.
According to Jay, one or two of them may still be readable, due to the fact their outer cases seem to have managed to protect the tapes within.
Entry 70 blah blah
Entry 71 blah blah
Since Alex is seen setting aside a box of tapes that were not given to Jay, it is possible that the tape found at the red tower was also one of these tapes, though Alex chose not to destroy it. ~Zarggg