Why this matters
In interviews and team meetings you will hear words like "unit tests", "UAT" and "shift left". Knowing the ideas behind them lets you follow the conversation, make sensible choices about what to test, and explain your choices to others.
A few ideas every tester relies on
Finding problems early is cheaper. A confusing rule spotted while the feature is still a written description costs a short conversation to fix. The same mistake found after release can mean angry customers, refunds and an emergency update. That is why testers read plans and ask questions before code exists. Teams call this "shifting left", because testing moves earlier on the project timeline.
You can never test everything. Even a simple form with one amount field has millions of possible inputs. Combine that with different phones, browsers and network conditions, and complete testing is impossible. So testers choose: they focus on the features that matter most and the places where failure would hurt most. This is called risk based testing.
Passing tests do not prove there are no bugs. Tests can show that problems exist. They cannot show that none remain. When you report results, say what you checked, not "the app has no bugs".
Bugs tend to gather together. A part of the app that has produced several bugs often hides more. Spend extra time there.
Running the same tests forever finds less and less. Once a set of tests passes repeatedly, it stops finding new problems. Refresh your tests and explore new paths from time to time.