The short answer is yes; you have to respond to oppositions.
Once your trademark passes the examination phase, it will get published for opposition. Each jurisdiction sets its timeframes for the length of the opposition window. The owners of previously registered marks will have legal grounds to file an opposition against your application. This is usually the case if the owner of an existing mark believes that your application might infringe on their trademark and cause confusion in the given market. This is called filing based on a 'likelihood of confusion'.
If you receive opposition, you must respond within a specific period stated in the issued documentation. Otherwise, your might be met with a negative outcome, and your trademark might get rejected.
It is always beneficial to hire an attorney or a lawyer who can prepare the opposition response for you, as some legal requirements must be met. They might also help you negotiate with the other party.
The best course of action to take varies depending on the case, but there are generally three approaches to addressing opposition. You can find more about them in our article What happens if someone opposes my trademark application?