The strength of the signal of an analyte is proportional to its concentration. The lower the concentration, the lower the signal. Sensitivity means how high a response a certain amount of analyte gives. So an increased sensitivity means a higher response from the same amount or concentration of an analyte. It is important to know the lowest concentration of the analyte needed to get a signal that can be distinguished from the baseline noise. If the concentration of an analyte in a sample is lower than this limit, we cannot detect it even though it may be present in the sample.
Selectivity in analytical chemistry is the ability of a method to correctly determine an analyte without interference from other compounds. A method's selectivity is increased if it's very specific for the chosen analyte(s), e.g. via chromatographic separation or fragmentation patterns in mass spectrometry.