Step 1: Recall the definition of independent events.
Two events $A$ and $B$ are said to be independent if the occurrence of one event does not affect the probability of the other event.
Mathematically, for independent events we have
\[
P(A \cap B) = P(A) \times P(B)
\]
Step 2: Substitute the given probabilities.
We are given
\[
P(A)=0.3
\]
\[
P(B)=0.4
\]
Using the formula for independent events
\[
P(A \cap B) = P(A)\times P(B)
\]
Step 3: Perform the multiplication.
\[
P(A \cap B) = 0.3 \times 0.4
\]
\[
P(A \cap B) = 0.12
\]
Step 4: Verify the result.
Since probabilities always lie between $0$ and $1$, the value $0.12$ is a valid probability.
Step 5: Conclusion.
Therefore the probability that both events $A$ and $B$ occur together is
\[
P(A \cap B)=0.12
\]
Final Answer: $\boxed{0.12}$