It's a college freshman physics level problem. An object in uniform circular motion like a planetary orbit, or motion on the surface of a spinning earth, is in constant acceleration. This acceleration depends on the speed and radius of the circle.
At the poles, you're just spinning around like standing on a record player, so there's no 'radius' and the acceleration is zero. At the equator, you're moving in a big circle at a high speed and the acceleration is the largest.
There is also, of course, acceleration due to gravity.
Your
weight (i.e. how hard your feet press against the ground) involves the combination of these two accelerations, so it changes (slightly) depending on where you stand on the earth. (There is also some effect from the fact that the earth isn't a perfect sphere)
This website provides a little more info.
As does
Wiki.
(Oh, and also there are effects due to difference in elevation, and 'anomalies' like if a huge metal deposit is under your feet.)