Why is there a +R or -R effect?

[kay]

What I mean to say is that why are some substituent groups such that(during resonance) the pi electrons' delocalisation is directed away from it while some groups are such that delocalisation of pi electrons is towards those groups?
Can you kindly give some examples too?

 

[Suraj M]

+R groups have excess electrons like -OH, -OR, -SH, -SR, -NH2, -NR2 etc.
and -R have deficit of electrons like -NO2, Carbony group (C=O), -C≡N, -COOH, -SO3H, CO₂ , SO₂ etc.