A “red state” is one whose voters elect primarily Republican candidates.
It is the opposite of a blue state, which elects primarily Democratic candidates.
There are different levels of how ‘red’ a state can be. If a Republican candidate wins the vote in that state, that state has ‘turned red.’ If a state votes for a Republican in nearly every statewide race, it could be considered a ‘deep red’ or ‘dark red’ state (Alabama, Texas, Idaho, etc.).
If a state typically votes Republican but will occasionally vote for a Democrat, they are known as a ‘light red’ state (Indiana).
There is no hard and fast rule as to what makes a state dark or light red. Some people may consider North Carolina or Iowa a light red state, and others may consider them swing states. Conversely, some people may consider Georgia and Arizona dark red states, but others may consider them light red due to their potential to switch in an upcoming election.