React-dnd what does $splice do

Took me a while to understand. Here is step by step explanation in conventional arrow function

  moveCard = (dragIndex, hoverIndex) => {
    // list of cards
    let newcards = this.state.cards; 

    // dragCard is card we are dragging
    let dragCard = newcards[dragIndex]; 

   // removing this dragCard from array
    newcards.splice(dragIndex, 1);

     // insert dragCard at hover position
    newcards.splice(hoverIndex, 0, dragCard); 

    // update State
    this.setState({
      cards: newcards
    });
  };

It's basically an immutable version of plain splice functions such as

newcards.splice(dragIndex, 1); // removing what you are dragging.
newcards.splice(hoverIndex, 0, dragCard); // inserting it into hoverIndex.

Instead of directly manipulating the target array, those Immutability Helpers are helping you update the state by creating and assigning a new state.