Paste CSV with X values in the first column and one or more Y columns after it, set the point size, theme and trend line option, then download the plot as SVG or PNG.
The first column is the X axis and every numeric column after it becomes a Y series with its own colour. If your first column contains text labels instead of numbers, the tool falls back to using the second column as X so the chart still makes sense.
It is an ordinary least-squares line of best fit through the first series, drawn with its equation so you can read the slope. It is a quick visual guide to direction and strength, not a full statistical model, so treat it as a starting point rather than a conclusion.
The renderer caps the number of drawn points to keep the browser responsive and tells you how many of your rows are shown. For exploratory work that limit is usually plenty; for very large datasets consider aggregating first with the summary statistics tool.