Not quite the same as being humble or modest, "self-effacing" is literally to efface -- "to blot out; expunge; erase; obliterate; cancel; destroy" -- oneself before others. It means not drawing attention to yourself, being restrained, shy, quiet, coy. It means you don't denigrate yourself, but you don't inflate yourself either. You don't fish for compliments or point yourself out. You just let yourself be, and if others point you out instead, so be it.
Modesty is responding with "No, that person did much better." Humility is responding with "Thank you." Self-effacement is responding to "You did a good job" with little more than a nod and a smile.