You listen to a slow French sentence and know the words on the text, but they seem to glide over the spaces. Vous avez does not sound like two separate blocks of sounds. The s in les connects lightly in a sound that may have seemed less apparent in les and amis taken separately. This is liaison: a final consonant that typically remains silent will be pronounced in front of a next word beginning with a vowel sound.
The difficulty in the beginning lies in the fact that a familiar word sounds somewhat differently when placed in a sequence. For instance, you know the final s in vous is silent but, in the context of vous avez, this s will have a z connection. Similarly, the final s in les could also serve as a z when it occurs before amis. Someone expecting words to sound alike in every context could find themselves in front of a sentence before the meaning is even known. A better strategy is to look for small units of words in phrases rather than words floating in isolation.
Start with very short phrases as a preliminary to practicing longer stretches of sentences. Examples might be: vous avez, les amis, un ami, deux euros, nous allons. Listen to just one phrase, stop and say it, and say it again at a slow rate. Then, try reading just the two words individually and listen to how this changes the flow of sound. This difference makes your ear listen for something new. The final letter on the written page had not been placed by some random event but, rather, was present because of the next word that would follow.
It is also dangerous to assume that the final consonants are linked. It is possible to link consonants when it shouldn’t happen and this may be even worse for learning a new word or phrase. It makes sense for a learner to know that some kinds of liaison are important, particularly in the very early days of learning, such as liaison with certain pronouns, articles, numbers, and words commonly used in simple everyday phrases. You don’t have to learn every rule at the same time. You could simply note the liaison when you hear it during practice in a course of French phrases or slow audio. You can record, for example, a note such as: “vous avez sounds like vou-z-avez; or les amis sounds like le-z-amis” in your personal phrase dictionary.
The good news is that, if you recognize the presence of liaison, it makes understanding listening even better. This is because of the fact that a sentence such as Vous avez une table can sound like one big word but you know that vous avez one phrase and une table is another phrase. This is how the sentence can be made out: vous avez and une table. This strategy, however, will help you hear the parts of a sentence more easily by repeating a small segment of a sentence until you can clearly understand it. Then move to the next segment. This allows you to understand the listening segment in a better way than listening to the whole sentence several times in a row and waiting for the pieces to become clear.
An indicator that you can understand liaison in the spoken language is to hear the sound without speaking too clearly. In other words, liaison should never make short beginner phrases sound like they have too much sound in them, especially if the speaker is French. It’s just a small bridge. If you are reading aloud, check the word that comes before the final consonant that may need to be pronounced. If you are listening to French, listen for this extra z, n, or t. It’s simply just one way to break long streams of sounds into something we can understand.