The small X on the pavement marks the spot where President John F. Kennedy was shot in Dallas, Texas in November 1963.
No matter what theory you subscribe to regarding his death, the fact was that a sitting US President was killed. Of course, Kennedy was not the first US President to be killed in office. Hopefully, he WILL be the last.
There is a lot of history in the East End of Dallas, Texas. I highly recommend a visit at least once in a lifetime.