Every file on a computer uses a certain amount of resources when sent over the internet or stored. Keeping mind of your kilobytes (kB) and megabytes (MB) can prevent problems and produce a smoother online experience.
Computer resources do have physical limits to their capacities, even if the idea of computer resources can be scaled up indefinitely. So we really want to think of the sizes of files in a tidy, minimalist way and thereby make the most of the resources we already have. Although most people nowadays seem to have internet connections which cope easily with audio, video and high-resolution images, it is worth remembering that many people do not. If care is not taken, it is possible to produce a large media file that actually conveys no more information to people than a file a tenth or a hundredth of the size.
Software packages that consume excessive memory and disk space for their function are sometimes called "bloatware", and one could apply a similar aesthetic to media files. For instance, making transcripts available on a web site might help people to find the information they are looking for more quickly than having audio or video interviews alone. Similarly, you might want to consider whether it's easier for people, including those with visual impairments, to read the date and time of an event from a text email, or to have to open a large PDF or image file of a poster.
A byte is a sequence of 8 bits (enough to represent one alphanumeric character) processed as a single unit of information. A single letter or character would use one byte of memory (8 bits), two characters would use two bytes (16 bits).
Put another way, a bit is either an 'on' or an 'off' which is processed by a computer processor, we represent 'on' as '1' and 'off' as '0'. 8 bits are known as a byte, and it is bytes which are used to pass our information in it's basic form - characters.
An alphanumeric character (e.g. a letter or number such as 'A', 'B' or '7') is stored as 1 byte. For example, to store the letter 'R' uses 1 byte, which is stored by the computer as 8 bits, '01010010'.
A document containing 100 characters would use 100 bytes (800 bits) - assuming the file didn't have any overhead (additional data about the file which forms part of the file). Note, many non-alphanumeric characters such as symbols and foreign language characters use multiple bytes.
Data Measurement Chart
Then there is the hypothetical "Googolbyte" which would be a number of bytes equal to a 10 followed by 100 zeroes.