ROI (Return on Investment) shows the percentage return on what you spent in a channel: revenue generated minus spend, divided by spend.
ROAS (Return on Ad Spend) shows how many dollars of revenue you generated for every dollar spent — a ROAS of 3x means $3 back for every $1 spent.
Both figures are calculated automatically once you've entered your spend for a channel — LeadSight does the maths from your actual attributed revenue.