Short answers, pulled from the story.
Interplanetary contamination is the biological contamination of a planetary body by a space probe or spacecraft, either deliberate or unintentional. It matters because Earth microbes carried to another world could confuse or destroy evidence of native life, and organisms returned from another world could pose unknown biological risks to Earth's biosphere.
Forward contamination is the transfer of life and other forms of contamination from Earth to another celestial body. Back contamination is the introduction of extraterrestrial organisms into Earth's biosphere, including infection of humans in space or on other celestial bodies.
In Cosmic Connection (1973), Carl Sagan warned that Mars might harbor pathogens which, if transported to Earth, could do enormous biological damage. In Cosmos (1980), he wrote that he would want to be very sure before endorsing a returned-sample mission from Mars.
The Mars Sample Return Receiving Facility, or MSRRF, is a biohazard containment facility proposed by NASA to receive, analyze, and curate extraterrestrial soil samples returned from Mars. It must be rated biohazard level 4 (BSL-4), filter particles as small as 0.01 micrometers, and is expected to take 7 to 10 years from design to completion.
In August 2019, scientists reported that a capsule containing tardigrades in a cryptobiotic state may have survived on the Moon after the April 2019 crash of the Israeli lunar lander Beresheet. Earlier, the Apollo program left behind contamination whose extent has not been fully analyzed.
A human carries roughly a hundred trillion microorganisms in approximately ten thousand species as part of the human microbiome, and these cannot be removed without killing the person. Sterilization is therefore not an option for crewed missions, making containment the only approach, and effective containment to the same standard as a robotic rover is very difficult with present-day technology.