Base case $i=0$: $dp[0]$ represents the number of ways to tile a $2 \times 0$ grid. This is an empty grid, which has no cells. There is exactly one way to tile an empty grid: by using no dominoes. So, $dp[0] = 1$.
Base case $i=1$: $dp[1]$ represents the number of ways to tile a $2 \times 1$ grid.
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