Acids and alkalis can be described as strong or weak.
This does not mean the same as concentrated or dilute.

The strength of an acid (or alkali) depends on how ionised it is in water.

What is a Strong Acid?

A strong acid is completely (100%) ionised.
An example of a strong acid is hydrochloric acid.
A strong acid has a pH of 1.

hydrogen chloride (in water)    hydrogen ion  +  chloride ion
HCl_(aq)                                    H⁺_(aq)       +          Cl^-_(aq) 

All of the hydrogen chloride molecules become
hydrogen ions and chloride ions when they
are dissolved in water (see examples of other strong acids).

What is a Weak Acid?

A weak acid is only partly (less than 100%) ionised.
An example of a weak acid is ethanoic acid.
A weak acid has a pH of 3 to 5.

ethanoic acid (in water)      hydrogen ion  +   ethanoic ion
CH₃CO₂H_(aq)                         H⁺_(aq)       +      CH₃CO₂^-_(aq)

Some of the ethanoic acid molecules become ions in water
but most of them stay as molecules.
The reaction is reversible (shown by the arrow).