Liaison means the linking of sounds or words.When we say a sentence in English, we join or “link” words to each other. Because of this linking, the words in a sentence do not always sound the same as when we say them individually. Linking is very important in English. If we recognize and use linking, two things will happen:
1. We will understand other people more easily.
2. Other people will understand us more easily.
The important thing in linking is the sound, not the letter. Often the letter and the sound are the same, but not always.
The most common liaison phenomena involve /r/ appearing in non-rhotic speech in post-vocalic contexts. A rhotic speaker will pronounce words like far as /fɑːr/, whereas a non-rhotic speaker does not pronounce /r/ at all unless followed by a vowel. E.g.
For rhotic speakers this is just because far has an /r/ in it, but for non-rhotic speakers it appears because the first word ends with a vowel and the second word begins with a vowel – the /r/ links the two words together. In such cases, [r] forms a syllable with the following vowel in connected speech and therefore occurs in a syllable onset – such syllabification across word boundaries is a general feature of connected speech in English. The [r] occurring in this context is usually referred to as Linking R, for the simple fact that there is <r> in the spelling.
For speakers of non-rhotic accents /r/ is not pronounced after vowels. However, in these accents, when words that are spelled ending with an <r> or an <re> come before a word beginning with a vowel, the /r/ is usually pronounced. This is linking /r/. In rhotic accents the /r/ is also pronounced when the words are in isolation so cannot be termed linking.
Intrusive /r/ also involves the pronunciation of an /r/ sound, but this time there is no justification from the spelling as the word’s spelling does not end in <r> or <re>. Again this relates to non-rhotic accents; rhotic accents do not have intrusive r. Like Linking /r/ Intrusive /r/ is found in word-final position in phrases such as law /r/ and order [lɔːr ənd ɔːdə], the idea /r/ of it, spa /r/ is in which [r] is inserted after the set of non-high vowels [ə, ɑː, ɔː].
Thus, link a final /ə/ or even /ɑː, ɔː/ to an initial vowel in the same sense group by inserting an r-sound even if there is no r in the spelling. The /r/ added in this way is known as Intrusive /r/.
For further information click here English Phonetic Course Liaison