Врач объяснил, что смесь уже не должна быть единственной едой, потому что пора вводить прикорм.

Literally, смесь means mixture. But in baby-feeding contexts, смесь very often means baby formula.

So in this sentence, смесь is not just any mixture. It means the infant formula the baby has been eating.

Объяснил is the perfective past tense of объяснить, so it presents the explanation as a completed act:

• Врач объяснил = The doctor explained / finished explaining

If you used объяснял, that would be imperfective and would focus more on the process or repeated action:

• Врач объяснял = The doctor was explaining / used to explain

In this sentence, the speaker is reporting one completed explanation, so объяснил fits best.

Here что means that, not what.

It introduces a subordinate clause after a verb of speaking or explaining:

• Врач объяснил, что... = The doctor explained that...

This is extremely common in Russian after verbs like:

• сказать
• объяснить
• думать
• знать
• видеть

So here что is a conjunction, not a question word.

Because должна agrees with смесь, and смесь is feminine singular.

So the form of должен changes to match the noun:

• masculine: должен
• feminine: должна
• neuter: должно
• plural: должны

Since смесь is feminine, Russian uses должна:

• смесь должна быть...

Together, уже не means no longer or not anymore.

Compare:

• уже = already
• не = not
• уже не = no longer

So:

• смесь уже не должна быть единственной едой
  = formula should no longer be the only food

This implies a change from an earlier situation when formula was the only food.

Because the sentence means should not be the only food, and Russian needs the infinitive быть after должна to express should be.

Structure:

• должна быть = should be

So:

• смесь не должна быть единственной едой
  = formula should not be the only food

Russian sometimes omits forms of to be in the present tense, but here быть is an infinitive after a modal-like word должна, so it stays.

Because after быть, the predicate noun is often put in the instrumental case.

Here еда becomes instrumental singular:

• nominative: еда
• instrumental: едой

And the adjective must match it:

• nominative: единственная
• instrumental: единственной

So:

• быть единственной едой = to be the only food

This is a very common Russian pattern:

• быть врачом = to be a doctor
• стать учителем = to become a teacher
• оказаться проблемой = to turn out to be a problem

It means the only food or the sole source of food.

Russian uses единственный for only, sole, or one and only in the sense of there being no others.

So the idea is not that formula must disappear completely. It means formula should stop being the baby’s only nourishment, because other foods should now be introduced too.

Пора is a fixed impersonal word meaning it is time.

So:

• пора вводить прикорм = it is time to introduce complementary foods

There is no normal subject like it because Russian often uses impersonal constructions where English uses it as a dummy subject.

Other examples:

• Пора спать = It is time to sleep
• Пора идти = It is time to go

So пора works almost like a complete predicate by itself.

Вводить is imperfective, and it fits well here because introducing complementary foods is viewed as a process, not just one single completed act.

• вводить = to introduce, to be introducing, to introduce as an ongoing process
• ввести = to introduce as a completed action

In baby-care language, вводить прикорм is very common because parents usually start gradually: one food, then another, over time.

So пора вводить прикорм suggests:

• now is the time to begin the process of introducing solid/complementary foods

Пора ввести прикорм is also possible in some contexts, but it sounds more like focusing on taking that step as a single completed move.

Прикорм is a baby-feeding term. It means complementary feeding: foods introduced in addition to breast milk or formula.

It can include things like:

• purées
• каша
• vegetables
• other age-appropriate foods

So it is not just food in general. It specifically refers to the stage when a baby begins eating foods besides milk or formula.

In this sentence, that is why the doctor says formula should no longer be the only food.