What is a ten line syllable in a sonnet?

This is not really the right question to ask!

Sonnets are a poetic form in which...

There are 14 lines which in turn almost always (in English at least) have 10 syllables in them. These are arranged into 5 iambs. This is the technical description for a weak beat followed by a strong beat - it's a bit like your heart beat... de (weak) dum (strong). When you have 5 iambs in a line, this is called iambic pentameter. It's the same measure that Shakespeare uses through many of his plays.