A decision tree is simply a model where you follow branches and make a decision at each point. Like...

If we had tech support for a toaster, you might see:

    if toaster toasts the bread:
      if no: has turning it off and on again worked?
        if yes: great! you found a solution
        if no: hmm, try ...
      if yes:
        is the bread burnt after?
          if no: sounds like your toaster is fine!
          if yes: have you tried adjusting the darkness knob?
            if no: ship it in for repair
            if yes: try replacing the timer. does that help?
              if no: ship it in for repair
              if yes: yay you're toaster is fixed

Any chance you can ELI5 this to me?