Imagine the Earth was a transparent sphere (actually the shape is slightly oval; because of the Earth's rotation, its equator bulges out a little). Through the transparent Earth we can see its equatorial plane, and its middle the point is O, the center of the Earth.
To specify the latitude of some point P on the surface, draw the radius OP to that point. Then the elevation angle of that point above the equator is its latitude \(\lambda\) — northern latitude if north of the equator, southern (or negative) latitude if south of it.
How can one define the angle between a line and a plane, you may well ask? After all, angles are usually measured between two lines!
Good question. We must use the angle which completes it to 90 degrees, the one between the given line and one perpendicular to the plane. Here that would be the angle (90°-\(\lambda\)) between OP and the Earth's axis, known as the co-latitude of P.
On a globe of the Earth, lines of latitude are circles of different size. The longest is the equator, whose latitude is zero, while at the poles–at latitudes 90° north and 90° south (or -90°) the circles shrink to a point.