To quote the explanation on explainxkcd.com:
In this comic, Megan is visiting a website on her mobile phone. After loading it, the website asks for her location. The choice between allowing or denying a website or app access to certain information is common among smartphones. The term "location sharing" specifically refers to when a smartphone user shares their location with such an entity. An example is a weather app which would need your location in order to automatically find the correct forecast.
Megan is then asked her momentum, which she denies. The joke is based on the Heisenberg uncertainty principle, which, in quantum mechanics, states that some pairs of values cannot be known precisely and simultaneously. The most famous example of such values (and the example Heisenberg himself used) are location and momentum: the more precisely you measure the location of a particle, the less certain you are of its momentum, and vice versa.
Heisenberg's uncertainty principle is typically observed only at sub-atomic scales, and not at macroscopic scales (it is possible to measure both the position and momentum of a large object such as a smartphone or human being). Nonetheless, Megan refuses, actively enforcing the uncertainty principle as a conscious action rather than as a simple limitation of knowledge.
