What is bytes up\down in internet mean?

up transfer and down transfer, up going out of your device and down coming to your device. Hence upload and download... a byte is simply a measure of the amount of data, it is equal to 8 bits, i.e. 01101101 = a byte where bits are the '1's and '0's. A single byte is approx one letter so the word "apple" which is 5 letters is 5 bytes.