Liaison is a term that covers a specific pronunciation rule. Simply, the rule states that when a word ends with a consonant and the next begins with a wovel or a silent h (most French words beginning with h begin with a silent h, unless the word has a foreign origin), the final consonant joins the following vowel to form a complete syllable.
Un éléphant / an elephant is not pronounced un + ay-lay-fant.
Approximatively, it is pronounced un-nay-lay-fant.
Trop avancé / too advanced is not pronounced tro-a-van-say.
Approximatively, it is pronounced tro-pah-van-say.
When using the liaison rule, the pronunciation of some consonants is transformed when they are placed at the end of words:
The pronunciation of S and X becomes Z:
Les deux enfants / The two Children is pronounced (approximatively): lay-duh-zan-fan
Pris au piège / caught in a trap (approximatively): pree-zo-pyej
The pronunciation of F becomes V:
Dix-neuf ans / nineteen years (approximatively): deez-neu-van
The pronunciation of D becomes T:
Grand arbre / big tree (approximatively): granh-tarbr