trreplaces sets of chars, not words (mentioned due to duplicates).
echo 'hello world' | tr 'hello' 'goodbye'
echo 'hello world' | sed -e 's/hello/goodbye/g'
tr is for
transliteration, turning some characters into other characters. It doesn't match strings or words, only individual characters.
In this case, it transliterates h->g, e->o, l->d, o->y, resulting in the string "goddb wbrdd" instead of "goodbye world".
The solution is to use a tool that does string search and replace, such as sed.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.