Why there is Shada in sarir?
Because the lam in the definite article is assimilating to the sin at the beginning of the word. It's not al-sarir, but rather as-sarir, so the sin wears a shaddah to reflect this.
Why ال ?
This is a definite bed. In English, there is normally only one determiner. A pronoun like "this" is a determiner, so it "eats" the definite article, which is another determiner. In Arabic this doesn't happen, you have both هٰذا and ال.
Its a common thing in arabic when we are talking about our bed its "سريري" and when we are talking about someone else's bed its "سريرك" but when we only talking about a bed its "السرير"