mysql存储过程

有一个存储过程，其中有一段代码是这样的：

PREPARE statement FROM

"SELECT     p.product_id, p.name,

IF(LENGTH(p.description) <= ?,

p.description,

CONCAT(LEFT(p.description, ?),

'...')) AS description,

p.price, p.discounted_price, p.thumbnail

FROM       product p

INNER JOIN product_category pc

ON p.product_id = pc.product_id

WHERE      pc.category_id = ?

ORDER BY   p.display DESC

LIMIT      ?, ?";

我想问问
IF(LENGTH(p.description) <= ?,
p.description,
CONCAT(LEFT(p.description, ?),
'...'))
这部分该怎么理解？