Tactical voting means when you vote for someone you don't completely agree with, just because you want to prevent someone you really don't like from winning.
Imagine you have three candies, but you only get to keep one. Your mom tells you to choose one candy, and you really want the blue one. But you also don't want your brother to get the red one because you know he will give you a hard time later. So even though you don't like the green one as much, you choose it instead of blue because it's the best way to keep your brother from getting the red candy.
In elections, it's like choosing a candy. You might really like the person who is running, but you know that they don't have a good chance of winning. You also don't want the person you really don't like to win, so you vote for someone who has a better chance of beating them, even if you don't like that person as much. This is called tactical voting, and it's something people do to make sure that the person they really don't want to win, doesn't win.