What is this weird sword Icon that appears when I use a tool or weapon?

Here's what the Minecraft Wiki says about it:

Combat

• New “attack strength” combat mechanic:
  • A meter shows up after switching items or attacking.
    • Damage done depends on the fullness of the meter, with a quadratic reduction (attack does 20%–100% of normal damage as the meter fills).
    • Fills at different rates depending on the new attack speed attribute (see table on the changes to gameplay section for a complete list).
      • The haste effect will cause the meter to fill faster
    • Can be displayed next to the hotbar (left or right based on the player's main hand), under the crosshair or turned off.
      • Looks like the icon for the strength effect on the side of the hotbar, or like a dagger under the crosshair.
      • This is controlled in video settings.
  • A cooldown animation is displayed of the tool slowly being lifted up, when first switching to it.
• Plays sounds when attacked strongly, weakly, and parried.

You can also check the Mojang update article.