"Reading" is the process of looking at a series of written symbols and getting meaning from them. When we read, we use our eyes to receive written symbols (letters, punctuation marks and spaces) and we use our brain to convert them into words, sentences and paragraphs that communicate something to us.
Reading can be silent (in our head) or aloud (so that other people can hear).
Reading is a receptive skill - through it we receive information. But the complex process of reading also requires the skill of speaking, so that we can pronounce the words that we read. In this sense, reading is also a productive skill in that we are both receiving information and transmitting it (even if only to ourselves).