What is 'se' and how and why is it used?
I am a Rosetta Stone user. They don't explain why you have to use 'se' and what it means. The phrase
El hombre y la mujer se besan
has appeared and it translates as
The man and the woman kiss
I took the 'se' out of the phrase and it still translated as 'The man and the woman kiss'
The online translator has shown me that it doesn't matter if 'se' is in the phrase or not. Clearly there must be a reason why you need to use 'se' and I would like to know why and how it should be used.
Thanks guys 
2 Answers
In this instance the "SE" means that the man and the woman are kissing "each other". The Spanish love to know exactly what is going on. If you leave out the "SE" it means that the man and the woman are kissing but they could be kissing someone else. In English we appear to accept what is happening but the Spanish don't. For example, take the phrase in English below.
The ship sank.
In English we accept that it is the ship that sank. However, the Spanish cannot accept this. The ship sank what? Another ship by colliding with it? A bunch of swimmers who were too close? To overcome this anomaly the Spanish use "SE".
Therefore "el barco hundió" is too vague, however, "el barco SE hundió" makes it clear that it is the ship that sank.
Se is reflexive pronoun,third person,singular y plural:
LAVAR---LAVARSE (lavar + se) = TO WASH----TO WASH ONESELF
Me lavo = I wash myself
El se lava = He washes himself
Ella se lava = She washes herself
Ellos se lavan = They wash themselves
Ellas se lavan = They wash themselves
