Explain how lines of latitude and longitude people find locations on earth?

The line of latitude specifies where in the direction of North-South a given location is: it gives a line outside of which this place cannot lie.

Then, similarly, longitude specifies where on this line a place is: so the combination of the two gives an unambiguous "address" to a place: while there are numerous others on the same latitude OR longitude, no other places is at the intersection of the two - this makes the location unique and unambiguously possible to locate!