Ellipsis is a common grammatical device used to avoid repetition, make sentences more concise, and create a more natural flow of speech or text.
Note that when ellipsis is used properly, it does not affect the completeness or grammatical correctness of the sentence. Ellipsis is very commonly used, particularly in spoken English.

There are two main types of ellipsis in English:

Textual ellipsis happens when some parts of the text are left out because they can be understood from the surrounding context. One of the most common cases of textual ellipsis is omission of 'that' in a that-clause.

Sometimes, in coordinated clauses joined together by 'and', 'but', and 'or', words that are used in the first clause can be omitted from the second clause. For example:

The complement of a verb can also be left out when it is already mentioned and clear. For example:

In situational ellipsis, the omitted words are not necessarily understood from the surrounding text, but from the context of situation. Situational ellipsis typically involves omission of grammatical words such as pronouns, articles, and auxiliary verbs.

Situational ellipsis mainly involves omitting the subject pronoun 'I', especially at the beginning of a clause, because it can be inferred from the immediate situation. This is quite informal.

In informal conversations, third-person pronouns and dummy pronouns used at the beginning of a clause can also be omitted when it is obvious who or what the speaker is referring to. For example:

When the subject is the second-person singular or plural (you) or a third-person singular noun or pronoun, the auxiliary verb can be left out, especially in questions.

The subject pronoun (most likely 'I' and 'you') and the auxiliary verb at the beginning of a clause can be omitted when the meaning is obvious. This is very common in informal English, especially with questions.

In informal contexts, when articles (a/an, the) used at the beginning of the sentence are inferred from the context, they can be omitted.

In informal contexts, the first word of some fixed expressions or idioms can be left out, because it is understood from the context.

Apart from the two main types of ellipsis which are mostly concerned with the function words, there are other types of ellipsis that are less restricted to grammatical considerations and rely more on the context of utterance.

Gapping is a type of ellipsis that happens in the middle of the sentence (not at the beginning of a clause). In gapping, typically two elements of the clause (usually the subject and the object) remain after ellipsis.

When most but not all of a verb phrase is left out, pseudo-gapping happens. For example:

Stripping happens when everything, except a single element (usually the subject or the object), is omitted from a clause. The clause is often followed by words such as 'too', 'also', or 'as well'. For example:

Sluicing occurs in wh-clauses (clauses beginning with the wh-words such as 'who,' 'what,' 'where,' etc.). In sluicing, everything except the wh-word is omitted from the clause.

A verb phrase ellipsis occurs when the entire verb phrase is omitted, leaving only the auxiliary verb or modal and the subject.

A noun phrase ellipsis occurs in a sentence when part of a noun phrase (a word or group of words that function as a noun) is omitted.

Some verbs need an infinitive clause after them as their object. In some cases, the infinitive clause can be completely omitted, or reduced to a single "to" in case of to-infinitives. Look at the examples:

Ellipsis means to omit one word or some words of the sentence because they are easily understood from the context.

There are different types of 'ellipsis' some are advanced and some are easy.