You're a successful doctor. You're giving advice to a high school student who might want to become an doctor. You say this to explain what training and skills they will need.
You need a solid foundation in basic math and science.
When you're talking about people in general, you can use the word "you". So in the example above, "you" doesn't mean the listener specifically. It means anyone who wants to become a doctor.
The "foundation" of a building is the part of the building at the very bottom that's connected to the ground. For a building to be strong, the foundation needs to be strong as well.
People often compare education to a building: in order to learn complicated things, you need to understand the basic ideas really well first. To explain this, people say that you need "a strong foundation in ___" or "a solid foundation in ___".
"Math" and "science" are often grouped together. You'll hear these words used together a lot. You can also hear them in the other order - "science and math"
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