If something is bipartisan, it has the support of two political parties that normally don’t agree on much. You might read about a bipartisan plan to improve the school system where you live.
The word bipartisan is easy to figure out when you break it apart: bi-, meaning “two,” plus partisan, meaning "supporter of a party." So something that’s bipartisan involves two parties finding enough common ground to support the same thing. A key aspect of something that’s bipartisan is that the two parties involved typically hold opposing views about the best way to do things, so a bipartisan agreement is one that likely involved a lot of effort, compromise, and cooperation.