The simple answer is that the GPS receiver in your phone can be accurate to within 3 m to 15 m (about 10 ft. to 50 ft.) under perfect conditions.
On average, your phone's internal GPS receiver will have an accuracy between 5 m (16.4 ft.) to 50 m (164 ft.), depending on natural and/or man-made terrain features.
More precise GPS location fixes are possible, but not for the GPS receiver in your phone. The antenna in a phone is too small and of poor quality, even in a smartphone costing upwards of $600 USD.
Wi-Fi positioning in modern smartphones can help matters, but this service is not always available in rural areas. A-GPS is another service that does improve phone GPS, but it is next-to-useless on long stretches of open road through un-populated, or low population, areas of the USA - for example, along I-70 between Grand Junction, CO and the I-15/I-70 interchange in Utah.
You also have to consider obstructions. GPS radio signals will not penetrate metal, wood, earth, certain types of plastic, certain types of glass, the human body, rock, concrete, cement, or more than 1 to 2 millimeters of solid water. Even the leaves on trees can effectively block GPS radio signals or cause severe multi-path errors.
Fortunately, rain and clouds are almost never a problem. The GPS system was designed by the US Department of Defense to be operable in most terrestrial weather conditions.