Renko charts are price charts with rising and falling diagonal lines of boxes that are either filled or hollow. Renko charts are "time independent" charts that do not have constantly spaced time axes. Example of Renko Chart is shown above.
Renko charts have a pre-determined "Brick Size" that is used to determine when new bricks are added to the chart. If prices move more than the Brick Size above the top (or below the bottom) of the last brick on the chart, a new brick is added in the next chart column. Hollow bricks are added if prices are rising. Black bricks are added if prices are falling. Only one type of brick can be added per time period. Bricks are always with their corners touching and no more than one brick may occupy each chart column.
It's also important to remember that Renko charts may not change for several time periods. Prices have to rise or fall "significantly" in order for bricks to be added.
Hollow bricks are bullish, black bricks are bearish - that's the simplest interpretation of Renko charts. Renko charts may be most useful in identifying trends and trend direction. Because they screen out moves that are less than the Brick size, trends are much easier to spot and follow. In order to avoid whiplash periods, some people wait unto 2 or 3 bricks appear in a new direction before taking a position.