Definition: Non-overlapping
arcs with the same
radius and
center, sharing a common endpoint

Try this Drag one of the orange dots that define the endpoints of the two adjacent arcs.
The red and blue arcs will adjust themselves to remain adjacent.

Adjacent arcs are two arcs on the same
circle which

Do not overlap, and

Share a common end point.

This means they are, in effect, joined end to end to create a larger arc.
In the figure above, drag any orange dot. The red and blue arcs will adjust to remain adjacent, forming a larger arc whose
arc length
is the sum of them.

The length of each arc can be added together to get the
arc length of the larger arc.