An abbreviation is essentially a shortened version of a word or of a phrase, usually in a written context but in some cases in a verbal context as well. They are used in order to avoid having to continually write out longer words or phrases and can also be used to save time. In some cases abbreviations are used as a way to avoid having many longer words within a text, or in some cases within speech and can be implemented as a way to abide by modern standards. For example, the abbreviation LOL, is used as modern slang. It is not uncommon to hear abbreviations used in modern spoken language such as the term YOLO, which people will say during a conversation.
There is no set pattern to what an abbreviation should look like and in some cases, the individual author may even make up their own. You are unlikely to see a pattern but, with that being said, there are many common abbreviations which are standard within the English language.
Depending on the abbreviation, you might see a word in a shortened way, for example the word rehab is often used as an abbreviation of the word rehabilitation, this is common place and widely understood. However, in some cases, the abbreviation may come in the form of letters, which is also known as an acronym. An example of this might be YMCA, this is an abbreviation of the widely recognised charity the Young Men’s Christian Association, although it is rare that anyone ever refers to it by its full name as the abbreviation is standard.