Concept:
The distance between two points $A(x_1,y_1)$ and $B(x_2,y_2)$ in a Cartesian plane is given by the distance formula:
\[
d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}
\]
This formula is derived from the Pythagorean theorem by treating the horizontal and vertical differences as perpendicular sides of a right triangle.
Step 1: Identify the coordinates.
\[
(x_1,y_1) = (0,0), \quad (x_2,y_2) = (6,5)
\]
Step 2: Apply the distance formula.
\[
d = \sqrt{(6-0)^2 + (5-0)^2}
\]
Step 3: Simplify each term.
\[
d = \sqrt{6^2 + 5^2}
\]
\[
d = \sqrt{36 + 25}
\]
Step 4: Final computation.
\[
d = \sqrt{61}
\]
Since 61 has no perfect square factors, the result cannot be simplified further.
Conclusion:
\[
\boxed{\sqrt{61}}
\]