What is a swing state?

The electoral college system ensures that even though US presidential elections are national, the vote is decided by counting the number of delegates from each state.
By Curved Bracket - Own work, CC BY-SA 4.0, 

Battleground States

Most states are comfortably red (Republican) or blue (Democrat) - see here. This means that presidential elections are essentially fought over a small number of purple or battleground states which swing between parties in different election cycles.

Ohio was the classic example of a swing state. It voted for the victor in every election from Kennedy's victory in 1960 to Trump in 2016. 2020 broke this pattern when the state voted for Trump over Biden.

The key swing state for 2024 appears to be Pennsylvania. This went Republican 
Under the electoral college system a candidate can lose the popular vote but still become president. This was the case in the 2000 and 2016 election. The distribution of votes is as important as the raw numbers.

What is the 'ground-game'?

The ground-game is ensuring that your supporters voters get to the polling stations to cast their votes. This can involve providing transport or repeated reminder calls. It requires large numbers of local volunteers interact directly with the voters.