This is nothing special for power cards in particular - powers in the books share this notation as well.
The [W] means your weapon damage dice. Crack your Player's Handbook open to page 276 — the Attack Results section — and look at the right hand side.
As in the example it gives: if the weapon you're using for this power is a regular dagger, your damage dice is 1d4. If a power instructs you to roll 2[W], you roll twice that. For a dagger, that means 2d4.
If your weapon dice consists of multiple dice already, you still roll twice that for 2[W]. A Scythe's weapon damage dice is 2d4, so 1[W] means 2d4, 2[W] means 4d4, 3[W] means 6d4, etc.
If you're still getting used to the combat system and powers (i.e. most things you will use often), there's a few sections you should liberally refer to:
- The Combat chapter, on PHB page 264.
- The How to read a power section, which is located right near the beginning of the Classes chapter (since that's where all the powers are) and begins at page 54.
- Also, equipment blocks (or equipment cards) aren't quite the same as power blocks (and cards). They're explained in the Equipment chapter, right at the beginning of the Magic items section, under the heading Reading a Magic Item on page 224.
The "at level 21" bit means your power upgrades at level 21. A lot of at-wills do this (and so do some other powers), because they'll be with you the whole way and had better not get outdated. From levels 1 to 20, you'll be rolling 1[W] for damage, but as of level 21, you'll roll 2[W] instead. (That's the "increase damage to" bit.)