Relative strength (comparative) compares two securities to show how they are performing relative to each other. A security's price change is compared to another security's price change (the base security).

The relative strength is the pink line at the bottom of the chart. When the relative strength moves up prices are outperforming the base security (performing better than the base security), when the relative strength moves down prices are underperforming (performing worse than the base security). When the relative strength moves sideways both securities are performing the same.
For example if we compare Diageo to the FTSE 100, Diageo is the main security, the FTSE 100 is the base security. The relative strength is calculated by dividing Diageo's price by the FTSE 100 price.

In this example Diageo performed better than the FTSE 100 after 23rd January (rising relative strength).
Note: Do not confuse relative strength with Relative Strength Index (RSI) which is a different indicator.