You know that adding ال makes a noun definite. But listen carefully — sometimes the Lam (ل) of AL sounds clearly, and sometimes it disappears! Look at these two examples:
The 28 Arabic letters are split into two groups based on what happens when الـ is attached to them.
The Lam of AL is clearly pronounced. It stays silent with a Sukoon (ْ).
The Lam disappears — the word's first letter doubles (takes Shadda ّ).
Say: "Sun letters steal the Lam" — they pull the Lam into themselves and double up.
Moon letters are polite — they let the Lam stand peacefully on its own!
You don't need to memorise the list — with practice you'll recognise the doubling sound naturally.