To counteract is to work against something in an attempt to reverse it or cancel it out. You can try to counteract the neighbors' annoyance at your loud music by bringing them some homemade brownies.
When you counteract something, you act counter — or in opposition — to it. To counteract rising global temperatures, we have to reduce the greenhouse gases flowing into the atmosphere. In order to counteract the effects of a long sleepless night, you may want to drink a cup of coffee or tea. And a good way to counteract a limited vocabulary is to learn a new word every day!