To be explicit about why, for others, this means your shell will search for executables in a 'bin' sub directory of whatever directory you happen to be in BEFORE it searches your normal path.
This allows for common commands like 'ls' to be executed from ./bin, if they're present, instead of /bin (from your system).
Once you've done this you've opened yourself up to an attack where you download a zip from the internet, extract it, cd into the directory and type 'ls' and you may have potentially executed something from that zip which you didn't intend to do.
tldr - relative paths in your $PATH is a bad idea.
To be explicit about why, for others, this means your shell will search for executables in a 'bin' sub directory of whatever directory you happen to be in BEFORE it searches your normal path.
This allows for common commands like 'ls' to be executed from ./bin, if they're present, instead of /bin (from your system).
Once you've done this you've opened yourself up to an attack where you download a zip from the internet, extract it, cd into the directory and type 'ls' and you may have potentially executed something from that zip which you didn't intend to do.
tldr - relative paths in your $PATH is a bad idea.