First understand the architecture of the temple. There were three chambers. A large courtyard where a very large altar lay. An indoor lobby where only the priests could enter after washing and finally the Holy room which only the high priest himself could enter.
The Holy room contained the ark of the covenant. This room was shielded from the lobby by a large curtain, the curtain was said to be something like one hundred feet wide and one hundred and fifty feet tall.
Anybody that entered past the curtain into the Holy room would surely die, the high priest could only enter once a year.
When Jesus died, Jesus became our ambassador to God. No longer did we require a high priest to enter the Holy room and sprinkle blood on the mercy seat. Jesus died and once and for all completed this requirement.
Thus God tore the curtain in the temple and opened up His Holiness to the entire world, through Jesus Christ.