What does the phrase "Washington would set a precedent" mean?

This is a hard question to answer because it's pretty vague.  It would be helpful to have some context.  But here's some help for you:

To "set a precedent" means to do something that other people will be likely to follow when they are faced with similar situations in years to come.  So, for example, when the Supreme Court ruled that states couldn't outlaw abortion (Roe v Wade, 1973) they set a precedent and other courts have tended to follow that precedent.

If your question refers to George Washington, I'd guess the precedent he set was to not run for reelection to a third term as president. When he did that, he set a precedent and presidents after him followed that precedent because it had been set down by Washington, who was, of course, a hero.  No president ran for a third term until Franklin D. Roosevelt did in 1940.

I hope this helps.  If not, please restate your question.