The definite integral is one of the major concepts of calculus; its relation to solving differential equations is illuminated by the two forms of the Fundamental Theorem of Calculus which are often visualized with the supporting justifications using the area interpretation of the definite integral.
See Sensible Calculus:V.C The Fundamental Theorem of Calculus.(draft)

Nowhere in the standard treatments of the two forms of the Fundamental Theorem do mapping diagrams appear.

Theorem CIS.FTC.I. Fundamental Theorem of Calculus I. (Derivative Form):
Suppose  $P$ is a continuous function on an interval, $I$, with $A$ any number in that interval. For any number $t\in I$, define the function $S$ by $S(t)=\int_A^t P(x)dx$. Then for any $c \in I$, $S′(c)=P(c)$.
Theorem CIS.FTC.II. Fundamental Theorem of Calculus II. (Evaluation Form):
Suppose $P$ is a continuous function on an interval, $I$, with $a$ and $b$ any numbers in $I$, and $F$ is an antiderivative (indefinite integral or primitive function) for P$,$ i.e., $F′(t)=P(t)$ for all $t\in I$. Then $\int_a^b P(x)dx=F(b)−F(a)$.

An important property of the definite integral which is strongly related to these two forms of the Fundamental Theorem and used in the proof of the derivative form is
Theorem CIS.MVTI. The Mean Value Theorem for the Definite Integral.
Suppose $P$ is a continuous function on $[a,b]$. Then for some $c\in (a,b)$, $P(c)[b−a]=\int_a^b P(x)dx$ or $P(c)=\frac{\int_a^b P(x)dx}{b−a}.$

Before continuing further with this section it would be helpful to review the materials in CIS.EM.TMD on Euler's method.

CIS.VFTC. Visualization of the Fundamental Theorem of Calculus