why is e silent in verbs like jouer or crier but not the rest?

Hi Clara, 

What it means is that in the case of jouer and crier 

you say 

je cri- rai ( 2 syllables)

and not

je cri -e- rai ( 3 syllables)

and the same for jouer 

je jou-rai

and not

je jou-e-rai 

Whereas in the case of parler or manger 

You have 

je man-ge-rai ( 3 syllables) 

je par-le-rai     (      "         )

The difference is that the -e of these verbs makes a new syllable in the future tense 

This is not the case in the present tense -

je joue ( -e is silent)

Je mange (    "     )

 I hope I have understood your confusion and explained the differences.