Combo charts let you combine bars and lines (or areas) on the same chart.
Metabase will pick one of your series to display as a line, and another to display as a bar by default. Open up the visualization settings to change which series are lines, bars, or areas, as well as to change per-series settings like colors. Click the down arrow icon on the right of a series to see additional options:
To use a Combo chart you’ll either need to have two or more metrics selected in the Summarize By section of your question, with one or two grouping columns, like this:
Or you’ll need a question with a single metric and two grouping columns, like this: