Question

how does the universal indicator help us to know the strength of acid or base

Answer 1

1 The universal indicator is a mixture of several indicators.
2 The universal indicator shows different colours at different concentrations of hydrogen ions in a solution

Answer 2

universal Indicator is the chemical that reacts and colours the solution as alkaline, neutral and base accordingly. for light acids its yellow to light red colour while neutrals are green and light bases are light blue to darker blue. red and purple/ violet are extremes. the indicator however, do not change the composition of the solution.
Its used to calculate the pH of a solution. We now use litmus paper to find out the acid or base, but we cannot get the pH of the solution out of litmus test.