Interest is one of the weirder mechanics in Auto Chess games, but you have to know about it to do well. Interest acts like a bonus to the gold that you earn for finishing a round, in addition to bonuses like win streaks and losing streaks.
For every 10 gold you have at the end of a round, up to 50, you earn 1 extra gold in interest. That means that your #1 goal in the early-mid game should be to get up to 50 gold and stay above that number.
Remember that you take damage at the end of a loss based on the enemy’s champions. Earlier on, players have fewer champions and their champions are lower levels. That means taking losses early is safer than taking losses later — so you should save gold early even if it means your wave won’t be as strong as someone else’s. 5 extra gold every single round adds up quickly.