I will try to explain intersymbol interference
After analog signal is converted into pulses, these pulses needed to be shaped (pulse shaping). In theory pulses are rectengular, this shaping is done so make pulses similar to theory. Then we have now symbols. If one pulse effects(because of multipath transmission. etc) the consecutive one this will be ISI and to fix this equalizer is used.
i will be appreciated someone can add something or correct me if i am wrong.
And if someone explain working principal of equalizer