What does 助人者得人助 mean?
助人 = help people
助人者 = those who help people
人助= people's help
得人助 = get people's help

者 http://www.cantonese.sheik.co.uk/dictionary/characters/591/
[1] he who; those who
[2] a particle combining with some words to form adverbials

Some example terms with 者
死 = dead; 死者 = deceased (person)
長 = elder; 長者 = elderly (person)
偽善 = hypocritical; 偽善者 = hypocrite (person)
違抗規則 = defy the rule; 違抗規則者 = those who defy the rule (person)
Edition:
Example for  [2] a particle combining with some words to form adverbials-
&quot;兵者，凶器也&quot; 。
兵者=(definition of) army;
凶器也 = is instrument of destruction