Below is an indeterminate tomato. Notice my fingers point and two suckers. They are located between the main stem and leaf branch. They need to be removed.
Notice, below, my fingers no longer point to the suckers. They were cut off. What is left is the main stem and tomato branch. You want the indeterminate tomato to grow as one vine. It's fine to leave 1 or 2 suckers on to grow if you choose. It just requires more staking and managing. You don't want the tomato to get so bushy and full of growth that it becomes more susceptible to diseases.
This is what the tomato now looks like. The suckers are removed.